For very long documents Word can be very difficult to maintain. Anything that requires you first write in simple text and then compiles is preferred. I tried Skrivenr, but found it old, clunky, buggy and poorly designed. I was far more productive in LaTex, but encountered problems when converting to Word (which most publishers prefer). Sure I tried Pandoc, but maintaining flow between versions of my book was a small hell.

In the end, I believe that there is no easy solution.

Typst has excellent collaboration tools. Both Typst and Overleaf do not support Doc export, but pdf to doc is relatively easy.