Is there a simple way to change the runtime core voltage? the VOLTAGE variable in the firmware Makefile is just for io_configuration right? I can cut the traces and power externally if needed.

The Nucleo board setup is already powering the dev board externally. There is a programmable potentiometer on the Nucleo board that sets the voltage; the VOLTAGE variable sets the value on the potentiometer (I think I have that right; it's been a while since I used the Nucleo board setup).

Ok interesting, I've turned the voltage variable down to ~0.9V where I would expect failure and things seem to work which surprises me, so I assumed the variable was not used as I expected.

@Jesse Cirimelli-Low: I'm aware that one of the Nucleo board types has a different potentiometer on it. The Nucleo firmware code assumes on of the board types and has a translation between the voltage output and the potentiometer programmed code. That translation is wrong for the other type of Nucleo board. So you probably want to measure the actual voltage on the VCCD line.

Ok thanks I'll check

Yes, although the MAIN setting connects to the LDO on the development board, so you will need to do the usual board mod of cutting the trace between the pins on the headers connecting power and installing a 2-pin header. Make sure your external supply and the Nucleo board share the same ground.