Onyx are Androids. But you should debloat/silence them a bit before, same as Kindle/any vendor Droid. And batterx life is not that great.
PocketBook have a more 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 took a Onyx instead of a Kobo, because Kobo are locked down Androids. Though they are quite open in supported formats too.