Hi,
Perske is a very good brand, so is Yaskawa and they are both very expensive.
The VFD is for an asynchronous motor and step/direction is not possible, that requires a servo type motor.
The VFD and spindle are very good indeed, I suspect that if yo set to 20000 rpm it will do it whether you
believe it of not.
You could put an index sensor on the spindle and have Mach count it out to confirm what speed its doing.
I am inclined to think that your PWM to DC converter is not linear.
For the work you are doing and the size tools (bits are for woodworkers) you are using just set it to run at maximum
and leave it be.
Quite frankly I think trying to buggerize with that spindle is wasting your time, you could very much more profitably
work on a flood coolant or air cooling solution.
Craig