I'm thinking supports easily copying my EPUB and PDF files to it (and without treating them as second-class citizens), lightweight for carrying for miles, non-glare screen, e-ink, optionally some kind of lighting when starting to get dark, non-invasive (e.g., no phoning home to report on my reading, no advertising and dark patterns pushing me to a store), not so expensive I feel too bad if I drop it or it gets lost/stolen.
If it runs open source, that'd be ideal, but I don't want to make a hobby project of this. I want it to arrive at my doorstep, copy some books onto it in 5 minutes, and go for a walk and start reading.
OK if it's an older model that's still available on eBay.
I've tried an Epson AR headset, but tethering, battery, and weight just aren't there yet for all-day ereader glasses, although I think that is a very promising field that will be viable soon.
Kindle does not support epub without conversion.
I have a Kobo Forma. Waterproof, light, great physical buttons, an 8-inch screen, 4:3 aspect (literally same as original iPad mini). Most PDFs I can read in portrait, but the rest are fine in landscape.
Kobo's native PDF support is garbage (as is Kindle's), but the koreader app makes it a joy (multiplatform, for Kobo, Kindle, Remarkable, others). It can strip the margins on the fly, is very fast, and has every config option you can think of.
Multiple ways to sideload, wired or wireless. Bonus: integrated support for syncing Pocket articles.
Drag and drop files on to it, reads most formats, side-lighting with adjustable colour temperature, no adverts, good battery life, no built-in store (unless you side-load). About £200, although may vary depending on where in the world you are.
It runs Android, but I doubt you'll find a custom ROM for it. But you can fiddle with lots of settings.
Plenty of older models should be on eBay. Paperwhite 3 and Paperwhite 4 might be a good start for a used (but still supported and receiving software updates!) cheap option.
Other open source devices like the kobo seem really cool and I'd like to try one but my kindle is still working some years later so it's all I've used.
The UX of the Remarkable isn't great, and the hardware is somewhat inferior to Kindles, but it does the job.
Seems there are several newer models that will connect with wifi