I first started with reprogramming the PMIC chip too through a separate Arduino Uno.
Then I found there's a much simpler way, just apply 3.3V power, move some dip switches on the breakout board then you can burn the boot loader from the Arduino IDE. See Portenta H7 : don't brick your board! - #12 by henksb .
My bootloader wipeout story is at Portenta H7 bricked . It still happens occasionally to me when I run an STM32CubeIDE project and forget to burn the bootloader before I power it down.