I am a moderately heavy kindle user and have been since the second version they shipped. When I upgrade, I usually buy the best new model available. I am skipping the one with pen support because Amazon’s text autosuggestions are absolutely the worst I have ever seen - it’s like they’re just using a random number generator and not a predictive algorithm - so my current Kindle is the Oasis.
It is so far beyond any other one I’ve owned that they’re not really comparable. The backlight is steady and even with no patchiness. The text reads cleanly with no fuzziness around the fonts. It’s comfortable to hold, and because it just inverts very cleanly and automatically it makes it trivial to hold upside down if you change hands or roll over. My requirements for a case are that it makes the device easier to hold and prop up for hands free reading in bed. Any of the origami cases should do - I think they’re all very similar in design but I’d just go off the reviews for build quality.
That said, there’s a number of kindle books that cannot be read on kindle devices because the publisher decided to prioritize the formatting over the text, and those I have to read on one of my iPads. I still prefer the kindle for text only books because it’s lighter and easier to hold.
The oasis has a slightly different form factor so it might be worth checking out in person, but I went from skeptical to really appreciating the design.