Hi Rachel,
I read your article with interest because I am looking for an efficient approach to “write once, convert to many formats” authoring, too.
Have you tried AsciiDoc?
http://asciidoctor.org/docs/what-is-asciidoc/
It is touted as being as powerful as DocBook but without the XML clutter. Free and Open Source authoring and conversion tools are available.
Thanks
Oliver