The big models from OpenAI, Anthropic and Google all write well; the bigger difference is the workflow around them. A long book needs an outline, memory of what's happened, and a way to review changes, which a plain chat window doesn't give you.
In Author Magic
You connect the AI you prefer, including models that run on your own Mac, and it handles the outline, the story bible and the review of changes.
KDP's rules change from time to time. Check KDP's own help pages for the current requirements before you publish.