There's a surprising lack of discussion surrounding the fact that it's mentioned in several places that Gale was an archmage prior to the events of BG3, and a pretty talented one at that, naturally capable of all the schools of magic
By normal 5E rules, Archmages are at LEAST lvl 18 - which means he already had access to (and presumably mastery of) 9th lvl spells. He still seems to have Astral Projection down, at least, heh.
True Polymorph (9th lvl), when concentrated upon for the full duration of an hour becomes permanent*. No checks are needed on a willing creature. Traditionally beyond that, polymorph only doesn't work when used against shapechangers specifically - which vampires are tagged as (mist and bats etc). However vampire SPAWN are not. This important distinction is further supported by the fact in game normal polymorph works on Astarion to be turned into a sheep per usual.
Tldr, there's nothing preventing Astarion from being true polymorphed back into a normal elf who looks the same, but would once again be alive and able to frolick in the sun. Ironically this happy ending remains available to him only if he does not ascend, as True Polymorph does not work on True Vampires.
*I believe he will still be susceptible to dispel magic. But Larian's rightfully afraid of implementing that spell anyway. And nothing would prevent True Polymorph from being re-upped again later.
