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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
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.
The document reader or the physical e-reader? For the latter, PineNote is still in development stage. For document reading, you will have to use Calibre + some plugins to convert kfx files, which still can't be read by readers. You can also download the books directly from the Kindle store in other formats, but I don't know how that would do.
https://www.dedoimedo.com/computers/calibre-kfx-drm-guide.html
I erased some advertising files in my kindle and never connected it to the internet. Anyway, a libre reader would be cool
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.
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.
If you're reading DRM-free ebooks on an android device, Librera Reader is available on f-droid and it works well. I have been using it for several years and it has many more customization options than Kindle does. There are several ebook readers available for Linux. I use the one that comes with Calibre, but there are a few that are lighter on system resources. If you're looking for an e-ink device with FOSS elements, you may want to keep an eye on the PineNote as it develops, as noted by other posts.
Calibre can easily shift the format of DRM-free ebooks. If you have an epub, it can make it a mobi or azw3 file. A fun option is to convert a copy of your ebooks into txt format. Then you can read your books in less or in your favorite text editor! Before you know it, you'll be reading multiple books simultaneously in tmux over ssh!
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