I have found approximately 170 microfarads of extra capacitance, in
bits and pieces. Wired them in. Set the V/f curve to linear (setting
0). I set the max frequency to 20 Hz.

With that, the compressor spins, runs, pumps, and the motor goes to
33% of its proper RPM. DC Bus Voltage drops from 320 volts to
approximately 255 volts when the motor is in action.

So it is a little victory, sort of.

Trying to set frequency to 25 Hz results in the motor getting stuck
and stopped.

So, it seems that extra capacitance helps, but I need much more.

I have ordered extra capacitors on eBay (electrolytic type, rated at
500 VDC).

I hope that maybe 1,000 uF would be about right.

Perhaps, for conservatism, I will supply them from a separate
rectifier, I have a pair of IRKDL71-06S02 rectifiers. It would be a
mess and I would prefer not to do it.

i