I would use Open Core if I were you, just be very OCD following the Dortania guide for your chipset.
It really is not all that hard. Use the generic SSDTs et al.
If you do it right you should be able to test your spiffy new OC EFI on a USB drive within an hour or so to see if it will boot your Catalina installation before installing Big Sur.
I did it yesterday, updating a clone of a Skylake Catalina installation with a 580 (I think that is a better way to go than risk a working Catalina installation) to Big Sur, but I had prior OpenCore experience wasting time finding out how a Ryzentosh is useless because of what won't run on it in these waning hours of MacOS on X86.
In any event I can't find anything that doesn't work in Big Sur like it did on Catalina.