Can't install from Rocky boot media, and BOOTX64.EFI

A friend has a PC, ASUS Z97M-PLUS, happily running CentOS 8.x. It was originally installed in UEFI mode (with secure boot) using media ‘CentOS-8-x86_64-1905-dvd1.iso’.

I want to install Rocky (8 or 9), but it fails straight away with the error:

Invalid image 
Failed to read header: Unsupported 
Failed to load image: Unsupported 
start_image() returned Unsupported

I know this is partly to do with the EFI firmware being old, but I think it’s also to do with the “shim”, BOOTX64.EFI.

The Fedora people have a very clear explanation of the issue and a work-around at this link:

They are saying to use an older “shim”, but should I really be trying to bury a Fedora file inside Rocky, and even if I was able to boot, would it just break at the next DNF upgrade?

I would not try to use something from Fedora to try to make Rocky work. I would try to update the firmware first before having to resort to anything else.

The firmware is already at the highest level, it’s strange that CentOS is working, but not Rocky, I’m guessing the “shim” files are unusual, in that they are different between CentOS, RHEL, and Rocky?

The PC works well except for this issue.

I don’t mind having special boot media just for this one PC.