As always reference the LinuxCNC manuals and the LinuxCNC Wiki for more information.
Tuning a Torque Mode Drive
You may need 10% of full torque to even break stiction so a lot of P term (and D term) is needed.
Torque mode drive tuning is quite different than velocity mode drive tuning (and generally more difficult)
For a torque mode drive, I would run the servo thread as fast as you can since LinuxCNC is now running the velocity control part of the position loop
(2 KHz might be possible with the GA350E 4 or 5 KHz is possible with faster CPUs)
Then for starting PID settings I would follow this procedure:
I would add as much D term as you can get until oscillations set in then use about 1//2 of that value
I would then do the same for P
Then I would add FF2 to tune out the undershoot during accelerate and overshoot during decelerate
Finally I would add I until the system becomes unstable
beware: large low frequency oscillations will occur
Then I would reduce the I value to about 1/4 of the value at which oscillations started