Jon, practically speaking, how fast do you run your spindle when rigid
tapping with small taps?

FWIW, I do all tapping at 200 RPM in back gear. I'm sure you can go faster.

....
My current thinking about the brake is that the pneumatic system is
already made, such that that the brake engages with a delay, but
withdraws instantly. If so, then, the built in delay may be sufficient
to let the VFD stop the spindle.

I recently put a brake resistor on the VFD, and can stop faster than I
used to.

FWIW, I use the VFD and resistor to stop spindle, then bring in brake to hold it there.

Karl