[Quoth]

I think apart from the issue of headings losing the h tag (easily put back and both style and page break is correct) the sequence of extra Save As in docx (not editing docx) and convert to epub2 Calibre is far better than other approaches for a a novel. All of the paragraph styles are automatically recreated as the correct CSS, including headings. The occasional inline formats are correctly converted to HTML. The only issue is the missing <h tags, which can be fixed.
The ODT import/conversion misses some page breaks and adds spurious spans, some of which are wrong. It's far better than Writer2epub or later LO Writer versions built in epub export.

So I will continue doing and extra Save As in docx and converting that in Calibre. It's pretty quick to find the headings and restore the <hx tag. The work needed for any other approach is much greater.

This is the section for Calibre posts, so I'm not going to write much about Sigil. I think it's of more advantage for reference works, text books and hand crafted epub2 than a novel as the odt and docx import plugins don't automatically map all wordprocessor styles to css like calibre does. I have used Sigil, but Calibre suits my workflow and source documents better. I don't need to edit the converted epub at all for proofing or betareaders, only for final publishing. There are no tables, dropcaps, images with flowed text, or small caps to be wrangled. Rarely footnotes and my system for them works fine.

The calibre docx to epub2 is very clean. I'll copy my docx from Linux to Windows 10 and Word 2007 to see is it the Save As docx in LO Writer that's losing the <hx tags or the Calibre conversion.