I respectfully disagree. Astarion is finally happy with who he is, finally accepted that he's enough.
He has made no indication he has a desire to return to human life. He wants to be free and also wants to walk in the sun.
You fixed the first issue by killing Cazador.
The questline for a nonascended Astarion is that he discovers he's enough and worthwhile exactly as he is -even as a vampire spawn.
He wants the ability to walk in the sun as a vampire spawn.
A cure is not the answer. It's not even something he has ever indicated he wanted.
Curing him of vampirism would be a solution that gives him the freedom to walk in the sun, but at the cost of changing who/what he is.
It's counter productive to everything you spent the game teaching/telling him.
In the graveyard, he has embraced his circumstances and his life. He says goodbye to the memory of his former human self. The one who 'lay here, dead and buried'
He can be a vampire spawn and find a way to acquire a sunwalking ability or find an item that grants it.
A cure isn't necessary and isn't inherently the answer because it fixes the sun problem.
