I'm daily-driving iMacPro1,1 but during this experiment I've tried MacPro7,1 as well.
I just tried your proposed changes in the following order and with a reboot between each:
- Reversed the two kexts so the Graphics one is first
- Removed IOPCIClassMatch
- Removed IOProbeScore
- Removed both entries at the same time
All of these led to the same result:
I can boot into macOS without problem, the 6900 XT is show in System Information, but with the message "No Kext Loaded". The Device ID shown in System Information as well as in IOReg is always the original 0x73AF.
Meanwhile I can see RM,device-id containing the values we've been trying out (currently 0x73AB).