May be of interest to this thread - I created a script that drives the Kindle web reader, captures screenshots, runs OCR, and creates an ePub - this wouldn't be as good as the pixelmelt solution, as it requires OCR.

Overview: https://www.hotelexistence.ca/remove-drm-with-kindleocrer/ (there's a sample conversion at the bottom, so you can see if it works "good enough" for your purposes)

Script: https://github.com/raudette/kindleOCRer

In action: https://youtu.be/3-07wMCKlkw