@ntnsndr@ethanz I really hope you get a good answer here. I've looked for something along these lines probably every six months for the past number of years, and it seems confusing and complicated so I end up just using my phone with all of its compromises or a laptop.
@coatilex thanks for this. Why is it so hard to have a book without a store in it? My favorite ereader was the Sony reader before they added a store to it. Such a great device.
@ntnsndr@ethanz It's not a perfect match to your requirements but for ebooks I use Calibre on my laptop (available for Linux) and connect that via USB to feed epubs to my PocketBook eInk reader. The reader theoretically comes with its own shop but I have never used it and there are no ads. The only reason I sometimes turn on the wifi is to access books from my local library. But you can run this device without ever connecting to the internet without problems.