Set the VFD to RAMP UP, and do it as QUICKLY as you can, without putting too much a hurt on your mains.
Set the VFD to NO RAMP on shutdown... just drop out... do NOT attempt to control the stop whatsoever.
The compressor was designed around an across-the-line start/stop by pressure switch. The unloader for that compressor, in conjunction with the compressor head output volume (downstream of the exhaust valve) serves several functions, but during starting, the unloader trip oint, and that output volume provide a stroke or two of 'no compression', followed by several strokes of 'partial' compression load. After about 4 or 5 strokes, you've got full head pressure load.
You DO NOT want to ramp it SLOW, because this will run you through many strokes, and introduce head pressure load BEFORE getting the motor up to speed.
Next: lubrication. The compressor either has splash, pump, or differential pressure lubrication. Splash is obvious, pump is obvious. Differential pressure lubrication uses difference between pump outlet pressure and inlet pressure to siphon lubricating oil from the pump's sump, to it's necessary parts. IF you run a compressor TOO SLOW, it will not lubricate properly under splash or pump operation. If you run a compressor with insufficient differential pressure, it will not lubricate properly... so get it up to proper speed.
Finally... ramp down... there is no reason to utilize dynamic or dynamic regenerative braking- there is no useful energy recovered, and no advantage to control the stop of compressor. Slowing the compressor will cause the unloader to flutter, rather than disengage... AND... if your lubrication system uses differential pressure AND the unloader dumps, but you keep spinning the compressor, you'll be running it with crippled lubrication.
Just set it to ramp up with as much expedience as you can (I would shoot for no more than 5 seconds) and have it simply cut-off at shutdown... no ramp, no DB, just drop out. The parameter on my drives usually call this "coast-to-stop".
The advantage to the VFD in this application is INRUSH. The Across-The-Line start inrush will be really high, but using the VFD to ramp it, even a FAST ramp, will quell that inrush substantially.
If you REALLY want to quell inrush, you can alter the drive ratio of your motor (smaller motor sheave) and recalculate the motor speed required to re-establish proper pump speed, and then program the VFD for higher run frequency. This will reduce motor torque during the ramp.