There seems to be a few publishers missing here ..

• Baen books (https://www.baen.com/) have never applied DRM to their books.
• TOR (https://torpublishinggroup.com/) stopped DRM on their books about ten years ago.

Baen typically sells their wares directly but TOR are through the usual sellers but have no DRM. I'm not sure how this works in practise with Amazon's new "you can't get the files" policy, but they are probably in cleartext somewhere.

Yeah. They are often very locked down and lack features you would normally expect - especially connectivity and configurability. There's a Youtube vid where a guy gets hold of one and the hoops he had to go through to install his own OS and get it to run some games... https://www.youtube.com/watch?v=bRoRPiDOtUg