I think rogue is pretty much perfect for Astarion too, I just don't really like how full rogue plays. So mixing in swords bard gives his combat a little more variety (I am currently trying the assassin/gloomstalker thing and it's so dreadfully efficient, it's hardly fun.) along with a bit of "charm is just another spell we cast" for crowd control. Which I find is very fitting for him.
Gloom stalker/assassin is a typical companion class. Astarion is perfect for that. His job is to kill things in combat,I do the talking. It's a one trick pony - kill things quickly and maybe o en locks.
I'm usually bard, I just love that class and I think, it's a good main character class.
