If you use Anna's for everything, essentially all of the available e-readers work just fine as a usb-stick-with-a-screen-attached. None of them in my experience handle PDFs well, but I'm sure there are large format ones that do better.

I have a kindle, but have never used any of the amazon specific functionality and don't plan on it. Stays permanently in airplane mode. I have no complaints and find the software more "refined" but not exceptional. I just convert everything to a mobi file.

My partner has a kobo and it seems just as serviceable. Out of the box it supports more file types, but it can be iffy on formatting sometimes, so I've had to fiddle with some stuff in Calibre to make stuff display nicely. I'm sure sticking to epubs would resolve that issue though.

TBH, I find all of the mass market e-readers to all have pretty comparable displays. I used to use a 20 year old kindle and don't find newer ones wildly better. The tech seems pretty stagnant. You're usually picking between things like backlights or light-temp now.