I was also surprised that finding a way to allow Spawnstarion to walk in the sun again turned into curing vampirism. I thought, we'd find him a fancy ring or something less drastic.
It is strange indeed, especially since there is no conversation before that if he'd ever want to cure vampirism. The cure comes out of nowhere. It seems like the majority of epilogue decisions are hand-waved. Just like you cannot settle down with Spawnstarion, you also cannot travel with AA.
The way Astarion talks about vampirism (my affliction) and that he took Cazador's offer only because the alternative was to bleed to death, made me always think he'd prefer to not be a vampire. At least I cannot recall him ever talking positive about being a vampire. To me, 'find a way to walk in the sun' was a euphemism for 'find a cure for vampirism' all along.
The artbook said that he was looking for eternal life. In Jaheira's quest, he will also say that immortality is hard to resist. I think his story should have been a little different. Now it's hard to say whether he wanted it or not.