I have a Kindle Paperwhite and I jailbroke it to install KO Reader.

The only difficult bit was getting a version of the Kindle software that was rootable. To do that you have to make sure it doesn't get updates, which basically means not allowing it to go online. If you ever go online, no matter what else happens, it tries to update its firmware. I ended up letting the battery on mine drain and then tossed it in a drawer for a few months. Eventually it was out of date enough that I could root it.

I got lucky because there was a jailbreakable version relatively soon. It looks like the last SW version that could be fully jailbroken was back in January 2022.

Wouldn't it be nice if Amazon had actual competitors and you could buy an e-reader that just ran open source software?