So what was the method you used in the end? Mine seems to be a bit inefficient to myself even lol
At one of the corners of the Pro Micro are a couple of solder pads labeled "J1". On my Chinese Pro Micro these solder pads were not bridged, which they must be for a 5 volt Pro Micro. When I learned about this I bridged J1 with a bit of solder and then was able to flash it in Win 10 using avrdude.
The command line I used was the standard:
avrdude -p atmega32u4 -P YOUR_SERIAL_PORT -c avr109 -U flash:w:filename.hex
I had Device Manager open so I could watch the serial port, which changes (did for me, anyway) between "running" mode & bootloader mode. When I saw what serial port Windows assigned when the Pro Micro was in bootloader mode I just up-arrowed in the terminal, edited the serial port, shorted reset and ground once more, and pressed enter in the terminal.