Thanks Tjaekel for nice words. Your procedure looks much more advanced than mine, but I'll take the credit if it led to your procedure. Just to confirm, I followed the above suggestion that I made, which is arguably crude and simple: (1) Move both dip switches away from "Arduino Pro", (2) apply 3.3V from an Arduino Uno to the Portenta 3V3 pin, double-click the ON switch on the breakout board and I could burn the boot loader from the Arduino interface. No PMIC programmer needed. And I was so proud of my little DIY PMIC programmer.