I had a similar issue with Plane here:
If Copter uses a SCHED_LOOP_RATE of 400, maybe try setting SERVO_DSHOT_RATE to 5, which would give you an output rate of 2kHz.
You could also try setting SERVO_BLH_OTYPE to 6 to set motor outputs to DSHOT600 even if they’re not identified by the automatic motor mapping.