I have a plan B: wipe the drive. Install Big Sur. Send the disk back. Let the owner boot from it. Use the migration assistant to migrate from his current Mojave install.
Things he will have to do himself:
Upgrade BIOS and make sure CFG is off.
Put in the serials of his current Clover build. What is absolutely necessary?
Adapt USB stuff to his own needs
Unfortunately he might have trouble with some third party kexts as the whole allowing these kexts system is broken in BS.
Am I forgetting anything?