Fun fact: Savin deliberately misled us with Ryn's design doc which said you'd be able to remove the cage after the Palace of Ice, then made a big deal of asking but not answering what her reasons to stay adventuring post-dungeon would be. Sneaky, sneaky.
As for the power of a god, consider what happens when Keros frees Kiyoko, Kinu and any other kids you have. Do you
really trust him to not pull something similar on Ryn, on you or both in exchange for helping out someone who doesn't even worship him and whose people are kind of at odds with his?
I dunno, maybe Lumia would be willing to do you a solid since the boer'alvar
are her people and getting that cage off their queen would be perfectly fitting her whole divine portfolio of encouraging fertility, but it might be a moot point depending on whether Alissa's plot or whatever prerequisites there are to become one of Lumia's chosen is finished first. And doing it the hard way lets Ryn be empowered and will make the end result feel so much more satisfying.
Oh, pretty sure the cage would be wasted on Alissa too, because Kas
specifically uses the 'if you were a demon you could shapeshift yourself right out of that cage' argument to try and tempt Ryn. I think the best punishment for her would be forcing her to witness Ryn moving on with her life and being
happy.