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.
Any kobo products, second hand kobos are great as they’re really reliable.
Onyx is much nicer, essentially a tablet with an e ink display. Look at the onyx book 2 refurbished
I bought a used Kobo Aura in 2014 and used it until summer 2022. Then I bought a Kobo Libra 2 and my mother got the Aura and it still works flawlessly to this day. Kobos do last a long time.
I pulled out my 2015 kobo glo HD, hasn’t been used since 2017… still had a charge and worked perfectly.
Also, KOReader adds a bunch of extra features and functionality.
I agree with the Kobo recommendation. They’re great devices. However, OP specifically mentioned azw3, which is Amazon’s format, right? You’re not going to be reading eBooks from Amazon on a Kobo. Your choices are DRM-free ebooks, or books from Kobo’s serviceable, but kind of crappy, store. Also, in most cases, pdfs are going to be practically unreadable on any Kobo; you need something more in the format of a reMarkable to get usable pdf on an e-ink device. PDF was another requirement.
Kobos will read azw3 and it’s easier to put on a kobo than it is a linux image. Also Calibre does a great job of converting azw3 to epub format.