im not too familiar with the specialized document creation
tools; but i wonder, could this replace calibre?

This is more similar to Org mode and Org exports. https://asciidoc.org/userguide.html

Asciidoc is lightweight markup language older than Markdown, it exports to anything, a2x is program that exists in the system.

Asciidoctor is new implementation, and also asciidoctor-pdf should be there, it produces beuatiful PDFs without LaTeX.

I wish to say that both programs could be included in Parabola:

1. asciidoctor, it creates beautiful websites and documents, books, EPUB, etc. Example output is their website https://docs.asciidoctor.org/home/

2. asciidoctor-pdf, https://asciidoctor.org/docs/asciidoctor-pdf/ and here is example of a document and screenshot of PDF: https://asciidoctor.org/docs/asciidoctor-pdf/#an-example-asciidoc-document