Copter-4.2.1-rc1
With SERVO_DSHOT_RATE=4
SERVO_DSHOT_ESC=2
MOT_PWM=4 (Dshot150) no motors span on multiple attempts
MOT_PWM=5 Dshot300 all motors span on arm
MOT_PWM=6 Dshot600 1st attempt, no motors span, 2nd attempt, all motors span, rebooted, worked first time
MOT_PWM=7 Dshot1200 - not a flicker
So Dshot 600 seems to be working fine now, but no dice on 150 or 1200, not that either of those worries me!