[retiredbiker]

My method might seem a bit techie, but once set up it works like a charm. Used on Pop_os! at the moment.

I use LibreOffice and Sigil. I have a LibreOffice template set up to start the process--it has all the custom styles I would use in a book (of course more can be added as needed). Use this for the text, and no ad-hoc formatting except short italics or bold.

Then in the Sigil ODTImport plugin I have a css file that exactly matches the styles in the LibreOffice template, but with units of em (not pt or cm) for the ebook. Import the LO file using the plugin, and you get two css files in the epub, a default and the custom one. Delete the default one, and voila, you have an epub styled just like to LO document, and even with the same style names. If you have added some style, you have to add that into the epub, but that is about all.

You may need to diddle the metadata, TOC, and so on. Sometimes I use the Calibre editor for that; it makes some things easier than Sigil.

LibreOffice of course can do a PDF suitable for publishing on paper, no problem. The template actually can have the page sizes and margins for a 6x9 paperback as well as the styling stuff.