Calibre will intake a .docx and nativly convert it to epub or azw3, but you will get some readability assumptions. Also, current Sigil has very good import plugins for both .docx and .odt files. Depending on how old your Sigil is and if you would update...
https://github.com/dougmassay/docxim...ugin/releases/
https://www.mobileread.com/forums/sh...d.php?t=274536
Any of these methods depends for quality on you having the source document properly styled, of course.