Onyx are Androids. But you should debloat/silence them a bit before, same as Kindle/any vendor Droid.

PocketBook have a mire limited app selection (though there’s a Nextcloud app) and their SDK is old. But they’re running pure Linux and are excellent in openness and battery life.

I didn’t take a Kobo, because they’re locked down Androids. Though they are quite open in supported fotmats too.