I have an old 2012 Nook that has literally never been connected to a network: I bought it to sideload epubs and pdfs onto, and that’s all I ever did with it. I never got a Kindle because you can’t sideload pdfs (or couldn’t at the time) and I didn’t like that Amazon was already sneakily retracting/editing purchased digital content.
It could use a new battery but it still works fine, and I bet these Kindles will too as I recall seeing guides out there about how to jailbreak/root them. If you can disconnect your Kindle from all network access and make it sideload (or non-Amazon-connected load) only, you’ll still have a usable device, as well as all your local content. Fuck Amazon.