Astarion with his default trickster subclass trades melee effectiveness for a selection of spells. If you respec him to a thief, he can hit pretty hard, and there are some build guides out there that can make him (or any thief rogue) downright fearsome. This one looks pretty good: https://mobalytics.gg/blog/baldurs-gate-3/rogue-thief-build-guide/

I decided to just respec my favorite companions as needed to keep them in the party. So Karlach is a ranger now. She’s not as combat-effective as a rogue or bard, but it’s a lot less of a stretch for her background.