Try the OpenCore EFI attached below.
It has been configured for a B450/5500/Nvidia system.
The iMacPro1,1 SMBIOS has been changed, as the one you were using was being used by a real Mac or another Hack.
Replace your current EFI with this one, don't change anything in the EFI.
When you boot the system and arrive on the OpenCore Boot screen, do the following.
- Press the spacebar button.
- This will unhide the Tools and Drivers in the EFI. Select the ResetNvramEntry.efi icon.
- This will automatically reboot the system and clear any existing NVRAM entries, so only the entries in the new EFI are used.
- On the next boot select the Install Big Sur icon.
If the system kernel panics take a photo of the verbose text, so we can see what is happening and what needs attention.
I have assumed you have set your Bios to work with macOS, as listed below.
- Some of these options may not be present in your Bios, match up as closely as possible but don't be too concerned if any of these options are not available in your BIOS
Disable
- Fast Boot
- Secure Boot
- Serial/COM Port
- Parallel Port
- Compatibility Support Module (CSM) (Must be off in most cases, GPU errors/stalls like gIO are common when this option is enabled)
- IOMMU
Enable
- Above 4G Decoding (This must be on, if you can't find the option then add npci=0x3000 to boot-args. Do not have both this option and npci enabled at the same time.)
- If you are on a Gigabyte/Aorus or an AsRock motherboard, enabling this option may break certain drivers(ie. Ethernet) and/or boot failures on other OSes, if it does happen then disable this option and opt for npci instead
- EHCI/XHCI Hand-off
- OS type: Windows 8.1/10 UEFI Mode (some motherboards may require "Other OS" instead)
- SATA Mode: AHCI