@zwol I wrote a book in Docbook (SGML first, and then converted to XML later) about 25ish years ago, with a custom vim configuration. It wasn't pleasant. Yes, the tools were bad, but also it's just really sort of tiring looking at all the tags and the processing toolchain was dire.
I was using a LaTeX-based toolchain for the PDF generation, I don't remember details, but Sebastian Rahtz was super helpful when I ran into mysterious issues.
I wouldn't do it again. Different time, different me.