Sure you can, just get an older one. I'm very happy with my jailbroken Kindle 4 running KOReader. AFAIK re-registering won't brick it, you can still sideload just fine.

We should be normalizing a separation of device and ecosystem. These are for consuming books, it's not an awful inconvenience to sideload every 19 hours of consumption to queue up the next read.

Or you can Zlibrary Koreader Plugin[1].

[1] https://github.com/ZlibraryKO/zlibrary.koplugin