You do have a third option.
Astarion doesn’t ascend, you release Cazadors 6 spawn into the Underdark and kill the other 7000, saying they would pose too much of a risk if released. Shadowheart disapproves, Astarion agrees and even the Gur in the end will agree with you that this was the best option. Not a very feel good one though, ut makes a lot more sense.
