I don’t know how the current state of more modern models is, but Kobo devices used to be quite hackable, at least about 7 years ago when I got mine. They are already running Linux under the hood, and although they come with a proprietary interface it was not hard to install koreader on my Kobo Touch.

Here the instructions: https://github.com/koreader/koreader/wiki/Installation-on-Kobo-devices

Some Kindles can be jail-broken and used with KOreader: https://github.com/koreader/koreader But I recommend to only use it if your reader supports Wifi… transferring books via USB is a pain with KOreader.

Cheap Intel Atom/Core M tablet running Linux with your favourite document reader?

Pine64 also has the PineNote if you want an E-ink display.

https://www.pine64.org/pinenote/

This, but it’s not quite ready for release yet and also it’s per se an e-reader but rather a tablet with an e-ink display, which makes it more powerful but also could have its drawbacks.

https://github.com/joeycastillo/The-Open-Book

Then there’s this which is designed so that you build it yourself and to be relatively cheap, but you have to assemble it so if you don’t have the technical skills it might be an issue.

You might have a look at the Tolino reader, it’s not really libre, but is one of the few e-reader that support Readium LPC (it’s definitely better than any competing DRM, but it’s still a DRM)

Also have a look at The Open Book project, it explains how to build an e-reader on your own

You can also consider to attach an e-ink display to a raspberry with Calibre installed

I erased some advertising files in my kindle and never connected it to the internet. Anyway, a libre reader would be cool