@ingo @gullok fwiw I occasionally read up on jailbreaks and hacks for old e-readers (to use them as web clients mostly) and I own a boox that runs on their in-house android. But from all I gather, the niche is just too small to hope for a user friendly FOSS alternative that drives e-ink tablets. (I was burned by PineTab, too). You could root the proprietary android-OS and run linux, but then you are on your own. The most usable way to tinker is probably to debloat it: https://appsec.space/posts/onyx-boox-go-10.3/

Debloating the Onyx Boox Go 10.3
I was looking for an eink tablet to r ead books and take notes while I’m away from home. After adventuring in the eInk rabbit hole I decided to go for the Onyx Boox Go 10.3: a Black and White eInk Android Tablet with 300ppi that’s also good for taking notes, weighting only 365g! I was a bit concerned about this report from Mozilla so I decided to take a look at the device.