[nhedgehog]

I don't want to burst your bubble but:
Most of the apps you are mentioning are not prepared for high contrast reading on e-ink devices, resulting often (not allways) in gray on gray resolution pictures. To make this even more confusing this behaviour can be device or firmware dependend, so some devices tend to show menus in some apps in white letters with white background, others show them readable.
In my experience android eink devices can be satisfactorily used for what you are intending to use them but require often a lot of fiddling to be satisfied with the result. Some of this mentioned fiddling means finding a specific version of an app from 1-3 years ago others means changing the app itself...
Best get one into your hands and try it out some time before bying it.

BTW: Some kindles are hackable and they all have an android backbone so why didn't you free your device first and use it to its limits.