Should I update to Big Sur?
@, well it looks like Big Sur upgraded my Boot Rom EFI, which means I still have the same issues when I reverted back to Catalina... Does anyone have a solution to downgrade the Boot Rom EFI?
You can't since Apple removed the possibility to downgrade your EFI Firmware for security reasons.
First it checks if the firmware version you're trying to install is newer than the one you have and they are now encapsulated in a package with specific hash.
Then the SPI flash memory doesn't have write access before the kernel is loaded. You'll need an external SPI flash programmer to write to the SPI flash memory and it's not a cheap hardware nor a tool designed to be used with little knowledge of what you're doing.
Sadly the risk to brick and render your MacBook Pro unusable is real if you persist in this way.
For more information: Source