It just occurred to me that it may only be the serial that’s failing due to changing CPU frequency after initialization. I tried calling Serial.begin after HAL_RCC_ClockConfig and initially got garbled text, but with baud rate 9600 it is working with PLLN 16, PLLM DIV4, which should be 64MHz. But F_CPU still reports 16MHz.
I also tried connecting the motor and it still goes through calibration. But it’s a bit weird. The PWM is in audible range. I turned my servo tester knob and the motor started spinning at a fairly high constant speed, and wouldn’t respond to the knob any further. Had to pull the plug to stop it.