Supernote has two sizes, I use the smaller one as an e reader successfully. About the height and width of a largish novel, but much thinner, and it runs a very open android fork that lets you treat it as a USB, along with various cloud sync options including self hosting. It’s primarily geared towards writing, but it natively supports kindle app for library books and is pretty easy to side load other apps onto. My one big caveat is that it isn’t backlit which makes night reading a pain, but some random person on reddit just started selling a light that clips directly into the pogo pins on that back and it works pretty well, though it’s annoying that it isn’t directly integrated so it’s another thing to keep track of