Is there any form of mitigation for this type of scenario: rebooting a device that is in the AFU state into fastboot mode in order to gain access to the device?

First, I would like to clarify that this is not an issue associated with GrapheneOS itself, but rather with the way Android devices operate.

Consider a hypothetical attack scenario where a GrapheneOS device in AFU mode is rebooted directly into fastboot mode in an attempt to extract access keys from RAM. Is there any countermeasure that can be implemented - other than locking the bootloader - given that this type of attack occurs before the device's operating system boots up?