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.
I was thinking of using this guide to install LineageOS on Amazon Fire HD 8 tablet: https://frontpagelinux.com/tutorials/how-to-install-lineage-os-on-amazon-fire-hd-8-tablet/
Kindles are really hard to root. Use XDA Developers forum for this kind of thing. https://xdaforums.com/t/fire-hd-8-2018-only-unbrick-downgrade-unlock-root.3894256/, which is what that guide is based on, looks like it only works for the 2018 version and seems way more difficult and risky than most ROM installations. Also, that won’t install Linux, just a different Android version.