Also save edits as odt but EXTRA Save As in docx to add to Calibre to convert to epub.
Works better.
The Calibre docx import is more often updated.
Don't reload the docx to LO Writer, as that converts it always, only edit odt, even if first version arrived as a docx.
Different ereaders & apps treat headers differently, but indeed you can make it look the same with NO headers, just <p and classes.
Many apps and ereaders have a limit to levels possible esp in TOC. A max of 3 is good.
Apps are not like real ereaders.
Section breaks can be just by a larger margin etc of a first breaking paragraph with no first line indent. They don't need a fleuron or character(s), but if used then that has its own WP style and CSS.
body text should always use <p
<h only for headings, but only needed for ancient renderers that ignore css
You can build a TOC automatically in Calibre from docx using various methods other than <h tags.