At the moment of the decision he really wants to be able to walk in the sun, and it is one of his motivations. The fact he prefers something else later doesn't mean he still doesn't enjoy the sun when he goes out. It's also one vulnerability less.
Astarion likes to say a lot of things but doesn't necessarily mean them. For all we know, he'll change his mind later. How can he even cast the fog? Sounds like he's daydreaming more than being a realist in that moment.
His background was cut in some ways, but he's still a vampire, being powerhungry is in his nature. Even before he learns about the ritual he wants to control the cult and talks about world domination as a fun project.
Either lover or friend, you're still the person he's closest to throughout his whole life and he listens to you when he decides to abandon the ritual. You are still the driving force behind his decision. If you're his lover then of course he's doing it for you, because he knows you won't change your mind and help him and he's scared of losing you.
Ascended Astarion doesn't want to eat the astral tadpole and is chill with you not enslaving the main enemy at the end. He also knows power is not everything but he certainly enjoys it.
Wording means a lot and it's everything when you sign contracts. To me Tav sounds manipulative more than concerned about him. There were better ways to phrase the persuasion options.
