FR actually isn't the problem here -- in fact there is an exploit in the game that could eliminate his vampirism. Kill him, use Gale's scroll of true resurrection, and if Astarian wants to come back the taint from his soul is cleansed. In the original Raveloft module this was one of the possible endings. You could use true resurrection to liberate Strahd's soul.
But this is exactly what I meant when I said it is a problem... it's no redemption if all you have to do to attain it is some technical magic. The problem is that as long as Astarion is a vampire he can't have redemption, he can only have some sort of redemption forced on him. any redemption of that kind is not really a character arc but more of a side quest.
Actually there there is a moment in true resurrection where you are given the option to remain dead. The hag's threat -- I'll kill you and then raise you and kill you again -- isn't a real one because you can say "no, dead is working out for me, thanks for asking". So mechanically this would be like the remove curse moment with lycanthropy etc -- werewolves can reject their cure. So when the moment comes -- stay a dead vampire or fallow the light and again as weakened moral -- would be the symbolic moment when you decide to live new life or let the character move on.
Or Astarian could return as an evil mortal eager to get bitten again . . .
And @Phea is right -- Jander Sunstar is an exception -- but you are also right its a contradiction in the rules. In 2nd ed elves couldn't become vampires at all. Sunstar got the DMs at TSR to agre violate two rules so he could tell his story
2nd edition was full of contradictions.