As far as I can tell, the ending doesn't change if you transferred 100% of the spirits. You just unlock some new prayer beads. Which is cool, I guess, but kinda useless since I'm not going to be doing another playthrough lol. At least not anytime soon. Only took me a few extra hours to get everything I was missing, though, so I don't mind. It's been a while since I 100%'d an open-world game.