I created a system that builds EPUB books for Bookapy. I would appreciate you guys giving it a test or two and report any bugs or suggestions.
For now it's in alpha status. It accepts HTML, text, Markdown, and .docx files. Embedded Images (other than the cover) are not supported yet, but will be.
You'll need one file for the book's text (.txt or html) and a cover image (JPG, PNG and Webp supported - minimum width 1400px, minimum height 2400px). When you submit the info, the system will build the EPUB and download it to your computer. Technically, it's very useable to create books suitable for publishing on Bookapy. You just need to preview the result and make sure it's decent enough.
Please test it and let me know what you think.
https://bookapy.com/author/epubbuilder/
Eventually when it's done, it will be the default in the submission system. But of course, for more advanced EPUBs, you would be able to submit your own EPUB.
Please note that the system is built with efficiency in mind. So the cover will be optimized to a max size of 2400px height.