Surprisingly, Remarkable tablets, despite not being open source, you can do just about anything with. They allow root SSH access and the backend is a heavily stripped down version of Linux.

I've been writing an application to allow customizing splash screens over SSH/SFTP and it's actually been super easy to work with. The "jailbreak" scene is also super active, and the company has gone the opposite direction of most. They retroactively removed the need for a subscription to cloud sync on all devices, and seem to very much embrace the ridiculous things people have done with their tablets.

The device is also no nonsense and does exactly what it's designed to do extremely well and no more. No ads, no bloat, no constant internet connection. You could never connect the thing to the internet if you really wanted. Honestly one of the few devices I've bought in recent memory that I feel like I wholely own.

Two big downsides are no Bluetooth, and you need a modified hardware device to unbrick the device if you fuck up (jumping type C pins to put the device into recovery). Overall really solid and would recommend.