documents4j VS pandoc

Take a look at https://documents4j.com. I'm not sure if it checks all your boxes. I have never set it up my self, but know it to be reliable in a production environment.

wget https://github.com/jgm/pandoc/releases/download/3.5/pandoc-3.5-1-arm64.deb sudo dpkg -i pandoc-3.5-1-arm64.deb

Send your publisher a .docx file made by converting a .md using Pandoc:

https://pandoc.org/

I used to work in publishing --- Word docs were hacked at by editors since it was their standard tool, then they would get flowed into PageMaker, or Quark XPress, or Ventura Publisher, or Adobe InDesign --- except of course for the LaTeX docs.

For .docx, the converted file is fine, so long as it has all the text and bold/italics which the author wants.

pandoc

I think yours is the candidate for Pandoc[1] or something like Soupault[2]. But you will be doing the HTML/CSS writing yourself.

1. https://pandoc.org

2. https://soupault.app

pandoc has a feature called filters. These filters are small programs that can manipulate the AST of the document. They can be written in any language, but the most common language is Lua as its interpreter is embedded in pandoc, and it is faster compared to the JSON filter interface which is also used by other languages (You may wish to listen to the Episode 37 of The Haskell Interlude podcast, where Joachim Breitner and David Christiansen interview John MacFarlane, the creator of pandoc, where he mentions Lua vs JSON filters).