Okay, here's what I found that really worked for me. Going with David's idea of epub>txt and then comparing.
For the conversion from epub to txt I used this online converter:
www.epubconverter.com/epub-to-txt-converter/
Super fast and accurate.
For comparison I like this online tool.
https://text-compare.com/
Super precise and it has a 'next change' option (in between the compared texts) which really helps with a huge bunch of text.
The screenshot is kind of cluttered but you get the idea.