The moment ePub was a thing I wrote my own scripts, decrypting and converting my eReader.com books to ePub by way of html, wrestling with cover art and text encodings.
I have got eBooks whose cover art is stairing at me now in Calibre which are (ship builders paradox asside) bordering on 15 years old by my reckoning.
Though Europe seems to have quite a few which atleast have no integrated eBook store and are designed to work with third parties. Such as the Onyx Boox and the like.
I personally use an old Sony PRS model. Though ive flirted with the notion of getting the Remarkable Tablet recently.
The kobo alternate firmware took me by surprise, as they seem to have eliminated all but two or three binary blobs, which is honestly very unusual in this space.
Onyx Boox and Pocketbook may both be worth looking at too, though they ship with google services.
Price is quite reasonable compared to the Kindle international edition. This is on my #tobuy list now.