Actually, thinking about it, we know that the tadpole was manipulated by Netherese magic to delay ceremorphosis. We know that the tadpole protected Astarion from the sun. We know that the orb Gale carries is Netherese.

A nice option would be that maybe, through a group dynamic built up to this point, that Gale, should you choose to let him ascend as a god or maybe even just with the power of the orb (and that he lives, and blah blah the other criteria), would in theory be able to cast a spell to grant Astarion the ability to walk in the sun.

We have seen light magic that protects from darkness with the pixie magic and lanterns in the shadow cursed lands.

It wouldn't be far fetched that the light could be muted/negated to provide Astarion protection by a darkness spell.

It would be a nice option and tie right into established lore we know by the endpoint of the game. It would be something of a potential resolution, even if Gale just promises to research it. Even a spell that has to be cast daily and is more of a temporary solution, or even imbued into a ring or something provides some resolution and isn't the total disconnect we see at the end currently.

The point being, with what is already established, it could fit right in to add the option.