@afewbugs Maybe you can put KOReader onto your Kindle and continue using it? https://koreader.rocks/
Getting it set up is, honestly, a bit janky. And the design is sometimes a bit rough around the edges - but! - it has _all_ the options, displays all the formats and even has PDF-reflowing, for reading A4 PDFs comfortably.
@afewbugs ... it looks like KOReader supports all Kindles
https://github.com/koreader/koreader/wiki/Installation-on-Kindle-devices
Gosh, it is only a half-recommendation though. It really does seem a bit nerds-for-nerds with the instructions for setting up.
I KOreadered my Kindle with Winterbreak, it was easy. Highly recommend it!