I was considering iPad but would be happy not to get another LED a screen
There's something oddly hostile about e-ink device companies.
I'm curious what that means. Is it a USB-C certified port? If it is certified and has USB host or USB-OTG on the logos, then it better work otherwise they'd fail compliance testing. If on the other hand it is just a USB-C connector for charging and for client mode, then that's normal. Most low end devices don't have a USB host controller and therefore can only be a USB client (like a thumbdrive) and thus can't connect to another USB client like a USB keyboard.
> There's something oddly hostile about e-ink device companies.
What are "e-ink device companies"? You mean like Amazon? Are they hostile?
Kobo's probably among the better options. It's straight-up Linux, and has e-Book functionality.
Any Android-based e-Book reader / e-ink tablet will also be able to run a bookreader. From my BOOX experience, Onyx's native bookreader (Neoreader) has some advantages in addressing the display as compared with several alternatives I've also installed (Pocketbook, FBReader, Koboreader). OTOH, Neoreader also misses some functionality, so it's a bit of a toss-up.
Pine are coming out with a tablet, it's not officially released yet, though there are some developer / reviewer units that are getting shipped. Its specs look pretty good (many tablets are shortchanged on storage especially).
reMarkable is another native-Linux device. It can take a keyboard and software is installable (also via APT AFAIU). Lars Wirzenius of Linux fame has a good review.
There are several e-book reader / e-ink subreddits which are the best general forums for more information that I'm aware of.