One possible way to account for how Joseph Smith was able to dictate the long and complex Book of Mormon is to argue that he had previously written the book in his own time.
The issue with this explanation is that we have no evidence of any drafts or previous versions ever existing before the original manuscript which the scribes wrote. The final version was effectively the first draft. We would expect that a book the length and complexity of the Book of Mormon would require pages and pages of drafts, as Joseph tested his ideas and improved the book until it was complete.
If there were no drafts then Joseph would have needed to create the content on the fly while dictating which is an incredible achievement.
Why do we have no evidence of any drafts ever existing? Why did no-one ever mention previous drafts or any pre-work before dictation?