Thank you. I used the Raspberry Pi imager to create a bootable USB stick, which led me to the same error message you received during the boot loader. I edited the config.txt file to include "os_check=0" as suggested but trying to boot from that simply gives a black screen - not even any boot loader error messages.
I understand that there is no official grommunio support for the Raspberry Pi but if they claim it is available and provide a download, surely they can do better than this? At the very least, make it clear that it doesn't (yet??) run on a Raspberry Pi 5, which would have saved me several hours!