I’ve personally found great effectiveness using cloud of daggers maybe more than any other spell. Funnelling enemies through a passageway with multiple cloud of daggers cast on the spot is probably my current meta. Then I utilize thunder wave/ black hole to keep the enemies on the other side of the daggers once they make it through.
Another thing if found amazing success with is getting a party to strength 20 as fast as I can, which then I find 90% of enemies can be thrown off cliffs for instant kills. Throwing enemies off cliffs also only requires stealth checks where the body hits the ground and dies.
Lastly there is a fun little cheese strategy I’ve picked up that I don’t see anyone talking about. If you cancel an improvised weapon attack with an NPC before you deal damage, you can move NPC’s without angering anyone.
Any persistent AoE damage spells seem to be extremely good, especially if you can set them up in a choke point and continually shove enemies back into them. Hunger of Hadar, Wall of Fire, Cloud of Daggers, Spike Growth, etc.
Spells that reposition enemies can also take the place of shoves: Thuder Wave, Eldritch Blast with repelling invocation. You miss out on loot if you kill a boss this way, but sometimes the cheese is too delicious.
Spirit Guardians has been good to me. Sometimes I just have Shadowheart cast it and spend her action dashing through all the enemies on screen.
Would definitely recommend giving Astarion the Thief subclass for the broken bonus action economy. It meshes well with his vampire bite ability.
I’d say Thief just seems to be way to go for any Rogue. Surprise can be a little unreliable to get in the game’s combat system for Assassin and last I checked, Mage Hand Legerdemain still isn’t working for Arcane Trickster.
I respecced Shadowheart to have more strength initially, and a reroll later after finding certain gloves, and made her a tank with spirit guardian. Stays in range of other characters for more potent touch heals, can take a blow, and Spirit Guardians in the middle of a pack of a restraining CC spell, and it builds up the damage. Freedom of movement, or equipment that grants the same/similar are super useful for this strategy. That said, I have only played on the Balanced setting, so take it with a grain of salt.
Edited to add, Life domain provides juicier heals and access to heavy armor.