I think the problem with him just disappearing after he chucks the lightsaber is that the structure of the movie (and of storytelling we’re familiar with in general) would make his disappearance play as a mistake and not a subversion or open-ended variation on the story. People would simply think someone “forgot” to have him show back up.
Once redeemed, he’s got to do something while in that state. He doesn’t have to be in that state for a very long time, obviously, but he’s got to do something WHILE IN THAT STATE otherwise the story feels incomplete.
So far the best possible edit idea I’ve heard (which - again, suggests a lot of skill on the part of the person putting it together, like creating a Mustafar establishing shot showing Point A and Point B, from original pieces) is that Kylo comes back, joins in the fight with Rey, and everything plays out the same way it did before - except when Kylo heals Rey, she wakes up - and he’s completely gone already. We don’t see him again until later, where the wordless ending sequence becomes a crosscut between the two - Rey at the homestead, Kylo at Ahch-To in self-exile. She wanders around the homestead, he’s climbing up the island, etc. etc.
The idea of ending the movie (sans weird nosy old lady) with her looking at the twin suns, and him staring at his own set of them on Ahch To is a pretty solid one, I think. But it would necessitate someone finding footage of him walking around in nature or whatever, compositing him into Skellig-Michel footage, getting the right shots to match between the crosscuts… It’s a lot. Maybe you could take the footage from “Silence” or something like that - but then you have the problem of everyone knowing the end of this Star Wars movie is kitbashed together from a Scorsese film? Pluses and minuses either way.