BLDC - Single motor for a range of voltage supply with PWM
Yes, it is technically acceptable to use simple PWM control to provide a constant voltage DC bus and to then use separate 3 phase variable-frequency switching to control motor speed.
It is also possible if desired to combine these two functions on a single switching operation where a DC bus is provided that varies in voltage with mains voltage and the 3 phase motor speed control then varies the effective voltage of the resultant three phase waveform by varying the duty cycle of the final switching waveform.
Both approaches have good and bad points.

Having separate voltage control and speed control switches allows lower DC bus voltages but care needs to be taken that the bus voltage never exceeds component ratings during start up, shut down and transient conditions.

Having combined speed and voltage control functionality allows elimination of the separate voltage control switching stage at the expense of some control complexity.


Your proposed method should work acceptably well.