You could but Kobo devices expected you to log in with a Kobo or other partner account (Rakuten, FNAC, etc.) on first setup. This allows you to bypass the need for any account and just sideload books on. You could previously hack around this but it sometimes caused issues.

Same, I bought my Aura ONE because it worked with good old USB Mass Storage and could read ePubs without any intermediary steps, both right out of the box. It’s a nice little device.

Exact same here. But to get the ball rolling, I had to set up an account and connect exactly once. Throwaway mail-account and immidiately afterwards I disabled all wifi access. Not ideal, but has served me well enough for a couple of years, and it's a very nice reader.

I wouldn't use a Kindle if you threw it at me. Nobody's damned business what I read and when.

Same here. I bought a Kobo H2O last Feb. I've only ever loaded books from Calibre over USB mass storage.

I chose the Kobo for the device being easily root-able. My daughter reads on it but I reasoned that if she got tired of it I could do "cool Linux stuff" with it.

AFAICT you can't just rely on USB mass storage, it is expecting their app to be present. I think there's an edit you can make to their MySQL database and maybe a calibre plugin does that?