If you can get an Onyx Boox tablet used by a first adopter that it wasn’t right for, it has the best of all worlds. Open, Android, EInk, large screen, and can be used to bang out content and read email. Other than that I would just get a downmarket but new Android Tablet and use it as a dedicated eBook and audio book/podcast device. The screen isn’t ideal but you can get a stock android tablet for $150 bucks us and use Caliber on it along with all the typical android stuff. Hell you might even be able to get a Linux tablet that would fit your needs but cost a little more. But if you go tablet the devices tend to be a bit more open to sideloading.
It was the theme from The 6 Million Dollar Man for me.