[Quoth]

Libre Office Writer -> saveAs DocX -> Calibre -> epub2.
But everything, especially the usage of the LO Writer has to be "just right" or it's wrong! Must not edit the docx, only native odt and SaveAs an extra copy in docx.
Headings, anchors, links, paragraph styles must be correct. Character styles don't work except the automatic ones for links, but inline bold or italic without a character style is OK.
No snap to grid, register true or other stuff only suitable for PDFs / POD paper versions.
Tables and footnotes (end notes) are possible, but best avoided.
I'd only use epub / mobi / azw for fiction / novels, not comics or graphic novels or technical documents or text books. Images need testing on 4" phone app, 6" kindles, 7" Kobo and 10" tablet.
Any "new" style needs checked on the real targets to make sure what seems like OK CSS & HTML & Images actually does what I think.
I practice by exporting ebooks as RTF, import the RTF to LO Writer and apply my style template / rules.
I have two templates for ebooks, one is default. The version of Word I was using seems tricker on styles and docx than LO Writer (word 2002 with updates). Word 2003 and Word 2007 are supposed to be best.