This is a followup to the earlier discussion about tablets here [soylentnews.org]. The wishlist was:
-Easy to install (or pre-installed) LineageOS, or some other open source OS
-Easy to open up and repair (and mod) the device hardware
-Matte screen with dynamic range that allows the screen to be bright enough to use in the sun but dark enough to use while falling asleep.
In continuing my search I came across the Librem 5, planned for release in Jan 2019. This phone (unfortunately not tablet) looks very promising:
Librem 5, the phone that focuses on security by design and privacy protection by default. Running Free/Libre and Open Source software and a GNU+Linux Operating System designed to create an open development utopia, rather than the walled gardens from all other phone providers.
[...]
Runs PureOS by default, can run most GNU+Linux distributions
[...]
Hardware Kill Switches for Camera, Microphone, WiFi/Bluetooth, and Baseband
[...]
like all Purism products, the case itself will allow you to access the insides, and the battery will be modular and can be replaced with ease.
https://puri.sm/shop/librem-5/ [puri.sm]
Besides the smaller size, the only thing missing from the requirements mentioned in the earlier thread is (probably) the bright display. I saw that such displays do exist [androidpolice.com] but doubt they will focus on that feature here.
Perhaps after this phone is released they can simply scale it up to a tablet as well? Also having the "brightest screen" could be the killer app for an open phone/tablet even amongst people who do not care about privacy. I have no idea how difficult that is to implement. Is it just a trade off with battery life (something I do not really care about) or something else?