I did something similar: 1. Jailbroken my old Kindle to freeze the software version. 2. Use it to get new books from Amazon, and then import them into Calibre

I did something similar: 1. Jailbroken my old Kindle to freeze the software version. 2. Use it to get new books from Amazon, and then import them into Calibre. Calibre can then de-DRM them, with the help of the dr-DRM plugin. It only needs the Kindle's serial number to work. 3. In addition to Calibre Desktop, I'm also running Calibre Web with the same database (my book database is on a network drive). Calibre Web also has OPDS server support. 4. I'm now reading books from a reMarkable tablet that has KOReader installed. It also has OPDS support, so I can browse my library from it and download books as needed. 5. I'm also using Storyteller to align Audible books and the Amazon Kindle eBooks, synchronizing them. Stuff that doesn't work: 1. Position sync between audiobooks and physical books. KOReader does have a position sync protocol, but translating its position into an aligned position is not trivial. 2. Automatic audiobook alignment when new books are added. 3. I'd love to use a Kindle Oasis with 4G with my eSIM to be able to sync the reading position. This was _the_ killer feature of Kindles for me.