I am perhaps the only one with experience, who has done this, so...

At this time, after 3-4 trials, I have a 2.5 kW AC brushless servo as the spindle drive, at 1:3 belt drive, via HTD 8-30 belts, on a very strong, sturdy, rigid frame of approx 60 kg mass.
So I get 0-1000 rpm, with a C axis or "live cnc spindle".

Servo is 10.000 counts at 1:3 belt drive for 1:30.000 counts at spindle.
This is not rigid enough for milling.
And my 600€ belt drive 30 mm wide, 8 mm profile, taperlock pulleys, 24:72 teeth at spindle, setup is very, very industrial, so...

The setup is excellent for turning and lathe use.
I get 90 Nm at spindle.
Spindle motor is 10 Nm cont, 30 Nm peak (3 secs) x 1:3 = 90 Nm at all rpm.

I will use it as-is, until I switch to a secondary, likely a (double) worm drive or planetary at 1:10, at some point.

My experiences, working at this for 10+ years.
Steppers are no-where near accurate enough.
Even the best belt drives are not accurate enough.

HAAS uses a gear on the spindle, for C axis use.
But they use a class 9 ultra-precision gear for the C axis, only, switch it in-out, and the gear grinder cost 9M$, I have met the guy who sold it.

So my system is better/similar to simpson of cubestudio, and not good enough in rigidity for C axis.
I use a bigger/more accurate servo, and similar/bigger belts (his are also HTD-8, I think mine were wider).
And my lathe is a 12x, fwiw..
Hth.