Absolutely love my BOOX Palma. It's a Kindle but if the Kindle was the size of a small smartphone and full Android but still eink. It's basically perfect. I can browse Hexbear, read my newspapers, books, whatever, all super easily on a great eink screen and it's tiny. No complaints, and it's like $200.

Book Reader runs Linux. So does ReMarkable (which is insanely overpriced, but actually nice if you can find a used one for ~ $150). The ReMarkable even includes instructions on connecting to the device via SSH and you can pretty easily run whatever you want on it. Including just making the rootfs a git repo.

never heard of this before

I have the box leaf 2 and it does exactly what I want it to - let me copy pirated ebooks from my desktop via syncthing, and it doesn't do anything I didn't ask it to