Two other suggestions for creating or preparing epub documents:
1. OnlyOffice Desktop (free): It offers directly the creation of an epub document (File | Save as). This is a free office program like LibreOffice.
https://www.onlyoffice.com/download-desktop
2. LibreOffice has some extensions to create epub, e. g.
https://extensions.libreoffice.org/e...ow/epublishing