First of all congrats on getting your setup back up and running. At least you have a decent system now.Ok, so an update on this situation. I am back up and running on BigSur
It was all a bit convoluted. I did end up buying a 2009 Mac Pro, updated from 4,1 to 5,1 as it was the only reliable way to gain access to my BigSur drive. However, it was not that simple. No surprises there!
My BigSur is installed on an NVMe drive. There is, of course, nowhere to plug that into a 2009 Mac Pro. So I bought an adapter card. Plugged it in to be greeted with a message that, "this version of OS X does not support this drive format" or words to that effect. I had updated to High Sierra but that was as far as I could go because the graphics card did not support METAL. So what to do? Source a new graphics card which, as we know is not a cheap option right now and then update to a version of OS X that would recognise the drive or try another option?
I had an idea that if I were to put the NVMe drive into an external case and plug it in via USB it would get around the issue of the drive not being recognised, and happily, I was right. The Mac Pro saw it as an external SSD and I was finally able to access and edit the EFI. Problem solved...or not!
Even having replaced the EFI with my backed-up version, my Hackintosh would not boot.
So not only would it not boot from the internal NVMe but it would also not boot from an external USB!
As a last resort, I decided to try and boot from the NVMe when housed in the external enclosure. And voila! It actually booted into BigSur, all be it rather slowly.
I then removed the NVMe from the enclosure, fitted it back into my computer and, it booted! It has been fine ever since.
I don't know why it is now booting. I can only assume that having successfully booted from the USB it somehow repaired any damaged/corrupted files?
So the upshot of all this is that I have my system up and running again but I now also have a real mac that I can use to access my files should anything go wrong in future. I can update the Mac Pro so it will eventually run BigSur and Monterey but that is something for further down the line.
Anyway, thanks for all your help with this. It was good to have someone on the other end who could look at things and point out where I might be going wrong.
As to your Ryzen setup, part of the issue as to why it is not working properly is because you may be missing the crucial kernel patches for High Sierra for the Ryzen.
You will have to find the correct kernel patches from older EFI builds online for your AMD setup as the newer ones posted at https://github.com/AMD-OSX/AMD_Vanilla only work for Big Sur or Monterey. Once you have the correct one, you simply replace them under Kernel > Patch > Add of your config.plist (not forgetting to specify how many cores your setup has under the settings).
As to running Big Sur or Monterey on the 2009 Mac Pro, unfortunately your options are going to be limited. That's because the maximum OS you can upgrade to (officially) for Mac Pro 2009 is High Sierra. To get it to Monterey you're going to need to use Opencore Legacy Patcher.