After thinking about it a bit more, there may be a third option for completing the Kylo arc. Rather than 1: Achieving total victory or 2: Being killed, he could just walk away from it all. I imagine it could play out where Rey creates some new, stripped-down version of the Jedi without all of their rules and hangups, and Kylo could realize that he really doesn’t have any problem with this interpretation of the Force. He realizes that even after destroying everything that reminded him of the past, it still haunts him, and that he’s the problem, not everything else. He helps Rey defeat the First Order and return peace to the galaxy, then disappears from the story for good.