I did a book in rst and liked that it had cool admonition, import, glossary, and index features that made it better than markdown for me. Still hate the heading conventions.
I have a custom pandoc filter for callouts and index entries. None of the simple lightweight markup languages has complete support for writing a real book. Writing custom rst code is a pain (and no one else in the world uses it). (I say this as a 25-year Python veteran and as a docutils committer!)
Have used asciidoc, HTML, word, latex, and rst.
Markdown is the least painful of all. It's not perfect but the others are worse.
(My custom stack uses markdown (or Jupyter notebooks converted to markdown). Pandoc plus some custom filters creates typst (for PDF) or epub.)