I'm the same as you wrt expecting my e-reader to work offline, and got a Pocketbook Verse (~$115 on Amazon in the US) that I've been happy with. It supports Wifi but I've never needed to connect it to the Internet and it works totally fine. No accounts, and has an SD card slot so you can just dump all your books on a microsd, slot it in, and the reader will automatically index them and make them available in the default reader app. The web browser and "store" (not sure if it's an app store or a book store - never tried to use it) don't work offline, obviously.

I was about to recommend it to you with no shortcomings, but I just turned it on to check the model, and it greeted me with a popup letting me know that "Pocketbook works better on WiFi! Connect to WiFi to make the most of your device!". I've never seen that popup before in the months I've been using this thing, it was easily dismissable, and everything that you'd expect to work still works fine offline, but that did just sour me on its offline story a little.