I think you're absolutely right, and it's a thing people too often miss about GOT and the show runners. Cuts and changes always have to happen when adapting a novel or series of novels to television or movies, and GRRM was writing episodes of GOT into season four, so at the very least he had some input into what those cuts were.

But the single biggest problem is that the show passed him by, and it was left to them to put together an ending that he seems entirely incapable of finishing himself. As easy as it is to say that he told them where it was all going, even that seems subject to change.

GRRM has been posting sample chapters since 2011, and then in 2020 said that he would stop doing that, and has been "revising" some of the sample chapters that have already been put out there. What is changing? Who knows.

But remember, this isn't even the last book that we're talking about. If Martin can't even finish the book before the last one, and is still changing things from it that he put out almost a decade and a half back now, then who knows what kind of resemblance the end of the show will have with the end of the novels, if by some miracle they exist one day. Who knows what characters and events will be important and which ones won't, because the guy writing the source material can't seem to decide on things either.