Only took me 2 years to get around to this...Spent the evening tuning the motor. Right now I have it disconnected from the slew bearing, but the planetary gearbox is still attached to the motor. There's no easy way to disconnect it, as the motor and gearbox are in a riveted housing.
To summarize:
I'm using the AMC 12A8 brushed servo drive I got from Al.

Attachment 462744

It is operating in Current (Torque) mode.

I'm using a single ended to differential converter from CNC4PC, I seem to be getting 8 counts per motor revolution. So for 1 rev on the output of the planetary gear I get (234:1) 1872 counts. This is reduced further by the slew drive (78:1), so will have 146,016 counts per revolution of the axis.

Here's a video just showing the settings, plots, and a move. The first plot/move is done with velocity set at 240 (1872 counts per rev of planetary output X 7.7 revs per minute / 60), this was the end result after basically just increasing P gain. Next I increased velocity to saturate the output (then backed it off a bit) and played around with feed forward. Both have a 2nd order low pass filter of 500Hz Q=1.4. Increasing D gain didn't seem to have much effect.

https://youtu.be/N-_amzw0As4

I think these look pretty good? Or no?

Only thing I wasn't sure about was what do with the settings on the servo drive. It is setup to operate in current mode. To set it up in velocity mode would require a tach, the manual suggests that current mode is the preferred mode anyway. There is one on board potentiometer to set something called Reference gain:

Reference Gain
This potentiometer adjusts the ratio between the input signal and the output variable (voltage, current, velocity, or duty cycle). For a specific gain setting, turn this potentiometer fully counter-clockwise, and adjust the command input to 1V. Then turn clockwise while monitoring motor velocity or drive output voltage (depending on mode of operation) until the required output is obtained for the given 1V command. Turning this potentiometer counter-clockwise decreases the reference in gain, while setting this potentiometer in the fully clockwise position makes the whole range of drive output available. This potentiometer may be left in the fully clockwise position if a controller is used to close the velocity or position loops.

I have it in the fully clockwise position.

Tomorrow I'm going to attach the motor/planetary gear to the slew gear and hopefully the tuning doesn't change much. The good news is that I have the rest of the fabrication work done. So provided that things still work ok once attached to the slew gear I can bolt everything together and try it a load on it. Just waiting on proper wire for the motor, should be here Monday.