As it stands, Astarion's "good" ending - and specifically, his non-romance ending, as I didn't romance him - is bizarrely bad. All that happens is he runs off crying, being burned by the sun, while your companion's are like, "Lol, what an idiot". You spend all this time trying to gain his trust, and push him towards not being an egotistical person who uses people for his own self interest, and this is the end? Quite literally all you have to do is:
1) Not have the companions crack jokes about it. What's up with that anyway? It pushes it from being kind of out of place and bad, to just being actively mean spirited towards his character, and the growth he's seen over the course of the story.
2) Instead of him running off crying, just have him directly reference the fact that the sun is rising, and because of his choices, he'll now have to go back to living like a regular vampire spawn - just now free from his Master. Have him talk up about how he's going to live his life more positively, and not hurt people or whatever. Have your character be able to respond back to him, and affirm his choice, tell him that he deserves this as penance for what he did, or whatever else. Then have him leave with grace, accepting of his new responsibilities and life direction.
