In regards to the tablet, I’m thinking about buying an Amazon Fire HD 8 though I’m open to any and all recommendations.
As for the distro, I’m fine with Ubuntu or Linux Mint (or for that matter anything that you may recommend).
For the ebook reading software, I’m leaning towards using Foliate since it supports kindle, epub, and pdf formats, not to mention that the UI is great and intuitive.
reMarkable is also a good device, very light and you can enable SSH/root access with a simple toggle on settings. There are also entire repositories of software for it https://toltec-dev.org/ https://github.com/Evidlo/remarkable_entware
Here a few link that you might find useful:
I believe it is the most open-source thing you can get right now that is a finished product working and whatnot with eink.