Some additions

> Does this work with the new(ish) Kindle (KFX) format?

I don't think so? Older Kindles still use the AZW3 format, and it's still apparently present for all the books in the Amazon store including the newest releases. I have a 6-year old Kindle Oasis that can download everything from my book collection, it's jailbroken with OTA updates disabled. I know that there are also people now working on making a downloader that emulates a Kindle device.

KFX might be more difficult to un-DRM in the future. Apparently, newer Kindles have multiple (more than 10) decryption strategies and Amazon can selectively activate them for each book.

My backup plan is to build an AI agent that uses a Kindle reader app, clicks the "page turn" buttons, and OCRs the text :)

> I get a 404 for that one. I think this is it now? Looks like it expects an audiobook to sync with the EPUB.

Yes, correct. This is a tool that allows you to make your own EPUBs with media overlays. I'm using it on Audible audiobooks (easily downloadable) and Kindle-derived EPUBs.