Thanks for the detailed and well elucidated write-up. I don't know the in-depth rules of DnD 5e, but even as a casual player, I've found that my play style has been narrowed down to:
1. sneak my way up to somewhere high above the enemies
2. Astarion sneak attacks with bow
3. the whole party then rains arrows/cantrips down on the enemies
I don't even care about team composition and whether certain spells can complement certain strategies, because they all pale in comparison to what getting a height advantage can get you. Even Shadowheart with a crossbow is a pretty decent shot with height advantage and the target is not obscured in shadows. And if an enemy does get close, it's so easy to disengage and jump behind the enemy to backstab. Gaining Advantage this way is so powerful and easy that the part of DnD that encourages the interplay of different class spells and abilities is being left by the wayside as I make my way through the game.
So the effect this has on my gameplay is that casting support spells aren't as useful as direct damage spells when cast from a height advantage. Astarion is incredibly powerful since he has so many ways of gaining Advantage, either from height or from jumping and then backstabbing. He doesn't need any spells from the party to help him, other than an occasional heal.
It could very well be the case that this kind of gameplay is what Larian wants us to have, but DnD combat is much more than merely sneaking up a ladder and then raining down arrows/bolts/cantrips, no?