@afewbugs Do you have the books downloaded on something / somewhere already? If so, it’s a pretty simple import into Calibre with the deDRM plugin. At that point you can put or keep them on your permanently-airplane-mode kindle (to avoid Amazon screwing with it more), or get fancy and jailbreak the kindle (more effort but definitely possible). But the kindle should be able to keep working with something like Calibre putting the books on it. No immediate need to get a new reader unless you want to.
If the books aren’t already downloaded somewhere, that’s the first step and I’m not sure how to get around that these days.
@afewbugs I’ll add I bought an old kindle specifically to do this with, and while I jailbroke it for funsies, I didn’t need to for the calibre route to work.
I tried KO reader and found it absolutely unusable without a touchscreen and worse than the native kindle software. So I just read things using the native software and load the device via usb. Once jailbroken there’s a way to load it via WiFi but it just wasn’t worth the setup to me.