bumping holding current - how it affects torque?

[Jonathan]

Based on your new graphs, the peak power is at about 600rpm, yet you're currently running at 240rpm. So for the 8Nm motor you're currently getting 130W (@240rpm), but if you changed the ratio to 600/90=6.66, then you would have about 175W (@600rpm) available - so 35% more. That corresponds to 18.6Nm on the output shaft, which is a much bigger difference than you're going to get from changing to 80V from 68V. Changing the motor to the 12Nm version and leaving the ratio the same gains about the same percentage, but presumably at much greater expense...

I've thought about geared motors, fitted with a planetaty gearbox, but those seem to be rather expensive
[...]
Unless of course I've missed something which is quite possible.

2nd hand cordless drill motor and gearbox.

Edit: When selecting the motor and gearbox it's sometimes easier to think of things in terms of the required power output. If you need 20Nm at 90rpm, then look for a motor that can output a little over 20*90/60*2pi=188W, then see if a gearbox is available to match the speed requirement. In general higher speed motors are more efficient (and therefore compact) - hence why cordless drills use relatively small DC motors which operate in the 1000's of rpm.