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.
Install something like LineageOS and use Librera or another reader from F-Droid. You will have a much better experience. If you really want to ditch Android , maybe you can get Ubuntu Touch, but device compatibility is dicey.
I looked into it and LineageOS looks good along with Libera.
If you’re going this route, a Galaxy Tab S7 Lite supports LineageOS