One other bit of advice: Rust isn't the only game in town, others have managed to successfully use these boards.
It usually pays off to look at how others configure your device to help verify your settings are correct.
For example, the zephyr folks do not have the same power configuration for the two boards you mentioned:

https://github.com/zephyrproject-rtos/zephyr/blob/main/boards/arm/arduino_portenta_h7/arduino_portenta_h7_m7_defconfig
https://github.com/zephyrproject-rtos/zephyr/blob/main/boards/arm/arduino_giga_r1/arduino_giga_r1_m7_defconfig

Check the values they use against the chip reference manual and the board docs you have before trying them, or you might end up with another board you can't use