A Song of Ice and Fire author George R.R. Martin has said he's written 1,100 pages of next book The Winds of Winter, the same amount he'd written as of December 2022, while also revealing there are eight Game of Thrones spin-off shows being worked on.
This is crazy but he should enlist AI to help him write the rest of the story in his voice. That is the only way this gets done.
Then he can just create the outline he wants to use and have it write it, and he can edit it. He doesn’t even have to tell anyone.