He just needs to find a single resolution. Just picking an event at random: when Tyrion attains power (which he never really wanted). Maybe end there. Or end with the demise of the White Walkers.

Yeah, but that's where he's written himself into a bit of a corner by eschewing conventional narrative structure. His books don't have a protagonist. That's the consequence of the big break from genre norms from the very first book. So you can't get a satisfying ending by reaching the end point of a single character's arc. It would leave 90% of the other character arcs unresolved.

He could have hewed to conventional genre narrative structure in a way that would allow the White Walkers to be the culmination of the series....but he didn't. They're barely in the books, and they're certainly not the focus of the story - they're at most a background, shadowy threat that plays virtually no role whatsoever in the events of the novels. The written series is about the political maneuvering in the world, not a fight against the White Walkers. Five novels in, it's probably too late for Martin to make such a drastic pivot while still getting to a satisfying ending and without 'breaking' the tone and style of the series.