Save As an extra docx
Wordprocessing:
only convert docx to epub. Then convert from epub to any other format, except PDF.
For PDF export from LO Writer. Then you do the header, footer, page numbers, pagesize etc in LO.
Note if the heading levels use custom names, they are getting <p tags, instead of h1, h2 etc but will all have an ID if you inserted an index in LO, and start new files, so the Edit Spans & Divs will fix ALL headings in one command, if you add h1, h2, h3 etc to convert option for <p> in setup.
Use regex match on id of id[.]* and select re-insert entire properties.
The odt should be used for edit and don't edit the docx ever.
Note for LO you can edit the settings in Insert Index for styles to suit ebook and without the .......123 page numbers. Then for PDF version, import/overwrite a suitable template, do front matter, and first page of chapter page styles, delete and re-insert index.
For edit for epub: Disable hyphenation, no page registration, headers, footers and do foot/endnotes manually (double link to word(s) to allow easy tap). Small page size to see that images/tables might do. No page numbers on inserted index.
For PDF export: Real page size & margins (multiple styles), hyphenation on, headers, footers, page numbers, registration on.