A Loupedeck or another midi controller might be a good ergonomic option for controlling darktable whist being in a comfortable position. That coupled with a monitor arm might be a working solution. Unfortunately I’m not sure if digikam supports midi input.
Lightroom mobile, while not free(in both ways), is a capable processor and modern tablets have decent screens even if it’s not possible to calibrate them(I think), I guess you can always do further adjustments or corrections if needed on the computer later on.