For the uses you mention, I agree with you that the typical 6 inch readers are garbage. But larger e-ink displays can be quite nice for that use case if the software is good enough.
I have a 9.7 inch e-ink reader by Onyx and the hardware is pretty good. In that screen size many books with diagrams look nice and you can see a nice amount of information at once. Alas, the Onyx software is passable but could be much better. I wouldn't recomend this particular model for a demanding professional, but as I said , with the right software this kind of technology can be very nice.
Have a look at this video:
https://www.youtube.com/watch?... Imagine the software of the device shown in the beggining with the screen shown at about 2:35.