If you consider designing a custom NIO (carrier board)for Bedrock SOM, starting from Bedrock Cartridge is the way to go.

Bedrock Cartridge provides the following:

• Highly effective 1st stage thermal coupling (TIM0) to the Ryzen die to a copper heatplate.
  Coupling the heatplate to a heatsink/cold-plate is easy. Coupling the die is challenging.

• Provision for mounting NIO securely with accurate spacing.

• Easy mounting of SOM to enclosure / heatsink / cold-plate.
  This is challenging without the cartridge since SOM does not have mounting holes.

• Thermal coupling for SOM’s DC-to-DC converters

• Mounting of NVME SSD
  Not present on SOM itself

• Securing and thermal coupling for SODIMMs

• RTC battery compartment