Thanks in advancefor the trust, I hope I can be useful in some way.
First of all, even if you say you don't get the game right now, worry not : I've taken quite a lot to learn strategies for games I play, too. Everyone starts as a newbie, then grows until (s)he knows each aspects of the game (s)he likes. It's just a matter of time and dedication!
So, about expansions! I totally agree with your starting strategy : the fewer expansions, the better. It makes you understand the dynamics of the game itself and the changes each expansion brought to it. What I generally can say about how expansions changed strategies, is :
Chronicles of the Godslayer (CotG) : Game basics. Playing this expansion alone makes you understand the very basilar rules of the game, which are :
Banish as soon as possible : Banishing useless cards for the deck like Apprentices and Militias, that definitely are below the average on any card of the game, makes your deck work better, allowing it to make you only draw the cards you mostly want to use, that will usually be Mystics and Heavy Infantries during the first turns of play (approximately by turn 3 to turn 10) and other cards you've acquired from the Center Row (if you acquired any) (like Runic Lycanthrope, Emri, One with the Void, Avatar Golem and so on) during advanced turns. To do so, you'll need to acquire as many banishing Void cards as possible during the very first turns of the game. What you should focus your Rune expenditure on turns 1~10 are Arbiter of the Precipice, Shade of the Black Watch and Void Initiate. But be careful : remember to banish Arbiter of the Precipice and Void Initiate as well, as soon as you don't have any more Apprentices and Militias in your deck, since their power is really limited, without that feature (drawing two cards is fine, but forcing the player to banish one from the hand becomes pretty nasty, on the advanced turns of the game, since you only could lose Honor Points and power doing so, while gaining 1 Rune is exactly the same of an Apprentice, and that's not what we should aim to, since we aimed for their banish in first place). Shade of the Black Watch is instead still fine, since it works (and is worth) exactly like a Heavy Infantry with the extra feature that grants you to banish a card from your hand or discard pile just in case you didn't need it anymore (like a Heavy Infantry if you had many Power-gaining sources like Avatar Golem, Muramasa, Demon Slayer and so on, or a Mystic if you had many Rune-gaining sources like Landtalker, Snapdragon, Reactor Monk and so on);
High-costing cards aren't always the best choice : Many player usually prefer a high-costing card to a low-costing one, but that's not always the case : if a card like Burrower Mark II is in the Center Row, but you have the necessary Runes to acquire Oziah the Peerless, it would be a wiser choice to acquire Burrower Mark II instead of him, since anyone can ramp to 3 Runes, anytime, meaning that if you don't take it immediately, the player after you will. Instead, 6 Runes are quite difficult to reach, especially on the first turns, meaning Oziah the Peerless will stay in the Center Row for a while, and you'll have another chance on getting it in the future thanks to the other low-costing cards you've acquired before him. This isn't always like this, though : according to your kind of deck, to which cards you need more at the time, to the Center Row cards and many other variables, it's sometimes better to acquire a high-costing card as soon as you can. For example, if Cetra, Weaver of Stars and Master Dhartha are in the Center Row, acquiring her could mean you'll be the most probable player that will get Master Dhartha for free (since drawing three cards doesn't always mean you can actually make all the combos you thought to, even though it's often like that). Unfortunately, cases like these depends by many variables, and can't be teached. Each player should find his/her own strategy in that, seeing the links and understanding other players' powers and intenctions as well. Practice makes perfect!
Always acquire card-drawing cards, no matter what : Cards that allow you to draw cards virtually are translated into "exchange this card into another card you have in your deck at random, unless you know which card in on the top of it", since playing them don't decrease your hand size, meaning you drill through your deck, finding more and more cards that you actually may need in that moment. Cards that also allow you to do additional effects beside drawing (a) card(s) are a must-have (Seer of the Forked Path, Flytrap Witch, Kor, the Ferromancer and so on). In first turns, Temple Librarian is a decent substitute to Void banishing cards listed above, if none of them popped up or you couldn't get them, since she makes you discarding a useless card (let's say an Apprentice) to get two additional cards that might serve you better. Banishing her afterwards, when Apprentices and Militias are banished, though, depends by the player : if you think she can still serves you well, you actually might keep her for her Honor Point value; otherwise, you should banish her as well, since you actually will draw better cards than her, in the future;
Get as many Constructs as possible, and as soon as you see them : Constructs are the best cards you can get and use in the game : you acquire them, you play them, and they stay on the playtable forever (unless pests like Sea Tyrant and Corrosive Widow are defeated, but, even in that case, nothing severe occurs, since you still can draw and play them again in the future), activating their effects each turn! It's like you have them in your hand each turn, but without occupying hand's spots! Mechana Constructs are the most Faction-dependant ones, but this handicap is compensated with the perfect Rune cost-Honor Points values ratio of 1:1. Furthermore, if you get enough of them, you also will gain many Power from Hedron Cannon, making yourself a Monster-killer, and getting Hedron Link Device allows you many, different combos with Burrower Mark II, Rocket Courier X-99, The Grand Design and so on;
Prefer Power-gaining cards to Power-costing-limited killers : I explain better : having an Arha Templar is surely good, because of his feature of killing a Monster having a Power cost of 4 or less. But what if no Monster are on the Center Row? The only one he could kill is Cultist, meaning you only gain 1 Honor Point. Pretty disappointing, isn't it? If you instead had something like Demon Slayer, you could had higher chances of getting a Monster in the Center Row (no matter its cost) by doing any other kind of combos before spending those Powers. It usually depends by decks, but Power-gaining cards are usually better than Power-costing-limited killer, since you can actually "dilute" your Powers as you like the most, while cards like Arha Templar and Oziah the Peerless have net effects (you play them, you use them, they stop there) and don't allow you later strategies (this is even more felt in the next expansions, while it still can actually work on CotG);
Be careful on the Honor Points Pool : As soon as the Honor Points Pool reaches ~20% of the initial value, you should focus your Runes expenditure on high-valued cards like Arha Templar, Arha Initiate, Void Initiate and so on, even if you feel they're useless : you still won't play long, and you probably won't draw them either, depending on how the match's going. Your judgment should be based on the Rune cost-Honor Points value ratio : a 50%+ rate is what you should seek for. That's why you always should acquire cards that cost 1 Runes : they always have a 1:1 Rune cost-Honor Points value ratio, just like Mechana Constructs.
Return of the Fallen (RotF) : The new Fate effect brings new strategies to your gameplay, especially related to your played cards' order and underlines the importance of not playing hastly. This means :
Only play the cards you actually need in a certain situation : Fate cards activate their effects as soon as they enter the Center Row. This means you probably will see them in action when a player defeats or acquires a card from the Center Row itself. Return of the Fallen's Fate cards have powerful effects, and you always should take advantage of them as many times as possible. Don't waste a chance of exploiting one of them just because you played all the cards in your hand before acquiring or defeating a card that left its spot to a Fate one! For example, if you need 3 Runes to buy Dimension Diver, just play the cards that give you 3 (more or less) Runes (let's say two Mystics), then see if a Fate card enters the Center Row. If it does (let's say Arbiter of Fate), you still have cards in hand to take advantage of their effects (for example, banishing an Apprentice or a Militia you have in it).
Be careful when deciding to acquire and use promoting cards : Those are like Arha Sensei. They let you acquire a Mystic or a Heavy Infantry, but they usually only fill your deck with them, in the long-term. You should try avoiding them as much as you can, but, if no banishing Void cards pop up in the first turns, cards that substitute a card from your hand (assume they're Apprentices and/or Militias only) with a Mystic or a Heavy Infantry are fine, too. Just remember to banish them as soon as you have too many of those two in your deck, according to previous strategies I've exposed;
Keep an eye on the Void : Return of the Fallen expansion also introduce Void-dependant cards like Serpentcall and Reclamax : they let you acquire or defeat cards in the Void itself, meaning you can defeat the same Monster twice or just acquire something you missed before. Knowing this, you also can use the Void as your personal, safe warehouse : if you acquired a Reclamax, for example, you'll likely banish Mechana Constructs from the Center Row as soon as they enter it (unless you can acquire them immediately or unless you have different plans about them), so you'll be the only one that'll acquire them from there when you mostly need them. Same for Heroes and Serpentcall. What instead concerns Sadranis, Dark Emissary, depends by your playstyle and your opponents' : since Monsters automatically go into the Void when defeated, Sadranis, Dark Emissary can both allows you to defeat a Monster from the Center Row twice, or just letting you kill the one you've missed or you're in need of at the time. Use him wisely, since 3 Powers are pretty limited, for a 7-Rune-costing Hero.
Did you ever dream about becoming the evil? When Samæl the Fallen is defeated, you can acquire Monsters like they were Heroes! And they give you crazy Power values as well! If you ever defeated it on turns 5~13, absolutely remember this : banish all your Heavy Infantries. The weakest Monsters give you 3 Powers when played, meaning they'll surely be better than them. If you instead defeated it later, you probably won't see as many Monsters as you would, meaning its feature is quite missed. Still, for each Monster acquired, banish a Heavy Infantry (unless the Honor Points Pool's running empty, of course).
Storm of Souls (SoS) : This expansion brings Events and related new standard Monsters, the Fanatics. Event cards are Center Deck cards as well, but, when draw, they globally stay active unless another one occurs, overwriting them. There are some new strategies as well :
Take advantage of the current Event effect, but don't force your playstyle to fit it : Event cards like Arha Rising have a really strong Event Trophy effect, and their normal ones usually benefit same Faction cards (except some of them). As soon as an Event pops up, try to take advantage of its effects, but don't acquire specific cards for that Event nor use their effects unproperly : they'll soon disappear, anyway, and your gameplay would be greatly disvantaged afterwards. Defeat a Fanatic as soon as you can, and keep him in your Thropy Room until an Event pops up, then consider to banish him to take advantage of it, if needed.
This is for keeps : Storm of Souls also brings Trophy Monsters as new Monster types. When you defeat them, they are placed in your Trophy Room and they stay there until you decided to use their effects, sending them back to the Void they belong to. Be wise, and use them only when you really need them!
Unite we stand : Lifebound Faction became overpowered in this expansion : because of the Unite Skill, the game now involves players to acquire as many Lifebound Heroes as possible, promoting a Faction-dependence beside Mechana Constructs one. Unite effects are usually very strong, and even low-costing Heroes can become very strong, when paired to other Lifebound ones. Definitely keep an eye on Lifebound heroes!
Destruction is only a mere step forward for the final project : This expansion also overpowers Mechana Constructs by giving them the chance to auto-destroy themselves. That means, you'll activate "Whenever you play a Mechana Construct" effects many more times than usual. And you also decide which you want to destroy! Definitely keep an eye on Mechana Constructs!
Immortal Heroes (IH) : This expansion includes Soul Gems, a new, common deck composed by Heroes of the previous expansions, like Arbiter of Fate, Master Dhartha and some others. Their powers swing between mediocre and overpowered, and you get them totally at random. Fortunately, not many strategies changes occured, this time :
Don't overestimate Soul Gems, but don't understimate them, either : Soul Gems are draw at random from the Soul Gem deck, meaning you'll never know which one you'll draw. Voids are the most valuable source of Soul Gems, followed by Driller Mark IV for Mechanas (combined with Hedron Pyromaniac, too). This means, don't play cards like Void Avenger, Spider Witch etc. starting with the assumption of getting something like Master Dhartha. Don't base your strategies on the Soul Gem you hope to get.
Rise of Vigil (RoV) : This expansion brings Treasures (Energy Shard), Center Deck cards (I think their number depends by how many cards are in the Center Deck, but I'm not sure about this) that aren't normally revealed as soon as they hit the Center Row, but they get covered by another card. When you acquire or defeat a card with (a) Treasure(s) beneath it, you also acquire all the Treasures under it. The Energize feature, altogheter with the Treasures' powers, bring many changes to the gameplay :
What do I really need of, right now...? Many Treasures will lay under some sucky cards like Monastic Tutor, Spirit Merchant and so on, but you might want them anyway, maybe because you've got something like Cognition Courier, Righteous Templar and so on, and that's better for you to have as many Energy Shards as possible. In this case, acquire them anyway. If you've got banishing Void cards already, don't even esitate and take them, since you can banish them whenever you want (as usual, unless Honor Points Pool's running out). If you instead don't have anything that can benefit of Energize, yet, it may be better for you to think twice about getting them. Strategies vary from players to players, so there isn't a right and a wrong one : just find your own one. Practice makes perfect, and players learn from their mistakes, too! I don't even need to tell you the more Treasures, the better, do I?
Darkness Unleashed (DU) : This expansion introduce the Transformation feature, that allows some cards to change their values and aspects according to (currently) the Energy value required. When transformed, they are worth more Honor Points, they have far better effects and, especially, they stay transformed until end of the game! Not many changes occurred in this expansion, though, fortunately :
What sucks now, will shine later : Cards that can transform themselves usually have normal/under-the-average effects, based on their Runes/Powers cost. When they're transformed, though, they have overpowered statistics for their costs! Cards like these are Honiskrot Tribesman, that transforms into Honiskrot Chieftan, Azerax of the Black Watch that transforms into Azerax, Nyx's Chosen and so on. Always get as many of these cards as possible, althogheter with as many Energy Shards as possible.
I unfortunately don't know anything about Realms Unraveled (RU) yet, since I didn't play it yet. I still know some of its basics, like the new Multi-Unite feature (oh God, that must be a blast), but I prefer to test it out first, before speaking.
I hope this could be of any help for anyone of you, guys!