I found the same problem with the disappearance of Writer2epub extension and Writer's built-in export of epub not being very good, a couple of years or so ago. I had also found that I preferred working in Sigil to Calibre because Sigil gave me cleaner coding. So I was happy to find that Sigil had plugins that can import odt writer files or docx.
The ODTimport plugin preserves all my styles in the CSS once a fairly simple config document is set up.
Check out the Sigil forum and its plugin index. ODTimport is well documented there.
https://www.mobileread.com/forums/fo...play.php?f=203