@CaseySJ thank you again for all the great and perflectly understandable content you create. Once again I was able to update to Big Sur like a charm while switching to OpenCore as you advise.
Just a question: do you still recommend to have extensions like Lilu or AppleALC to be also stored in the /Library/Extensions folder? I had struggle with them while trying to boot with OpenCore. Once I removed them it was ok. I was blocked with a kernel panic because of a kext version mismatch between OpenCore and the one stored there.
I removed all the extensions that I added in L/E & S/L/E and since then with the EFI folder you gave for OpenCore 0.6.3, everything works like a charm.