An good, small e-reader with an open ecosystem... it would be something! This Dasung Not-eReader is probably the closest you can get right now. When Readup reaches Android, it should be usable on there out-of-the-box.
And I'm drooling over the reMarkable 2 About page (must be one of the longest ones in the history of the internet, clearly taking inspiration from Apple). Why oh why must ecosystems be so closed. I suppose that managing an app ecosystem would only complicate things for reMarkable... It makes sense. But it sucks.