I have got my system working by following the advice in the above link to install BigSur on another machine. This relied on the fact that I have a very basic 13 inch MacBook and could install BigSur there.
Not a super clever solution, but a safe one - and I prefer it to options such as making risky patches to the BIOS:
- Buy a USB C to SATA adapter
- Take the OpenCore USB stick you have prepared earlier
- On existing PC / Catalina Hackintosh:
- Back up EFI folder from USB stick
- Delete EFI folder from USB stick
- Identify SSD to install BigSur on
- Disconnect the SSD from the PC
- On Macbook:
- Attach the above adapter to MacBook
- Attach the USB stick to the MacBook
- Connect the SSD to the adapter as an external drive
- Reboot MacBook while holding down option key
- Then boot the MacBook from USB and select the Install BigSur option
- In Disk Utility select to Install BigSur to the external SSD
- Reboot MacBook until installation basically complete, then shut down
- Go back to the existing PC / Catalina Hackintosh:
- Connect the SSD to the PC
- Boot into the existing PC / Catalina Hackintosh
- Copy the backed up EFI folder to the USB stick
- Restart the PC, select F8 and boot into BigSur from the USB stick
- Mount the SSD’s EFI partition and copy in the EFI folder
- Restart the PC, select F8 and boot into BigSur from the SSD
In case it's any use to anyone I've attached my EFI folder. It is Possible that a couple of SSDTs I created during earlier head scratching phase are redundant. Hopefully by the time I need to upgrade my system this will be a little smoother.
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