My son has been using Catalina on his 2009 Mac Pro (macpro4,1), firmware upgraded to a macpro5,1 over a year ago. Catalina and OpenCore 0.58 along with his RX580 4GB have been excellent. He can select any of his 4 disks at boot time through a graphical user interface and OpenCore helps with system updates by tricking Apple’s software into believing your Mac is no different than a natively supported machine.
With Big Sur now available and OpenCore at 0.63, he was ready to upgrade his Mac. We backed up his data and proceeded to follow these instructions, everything through Part I seems to be running fine. Will report back after we update through Parts II and III. Also, unless things have changed with the instructions above, this video found from this forum shows how to modify the EFI so that Big Sur can be updated – note, the instructions in this video should be reversed when you are done with Apple’s updates and you want to optimize speed of your Mac Pro. Also note, we were spared from having to mess with Clover to enable the onboard ethernet through the Intel Mausi Kext, I believe the config.plist file linked within the instructions above helps to enable this for Mac Pro hardware.
Last note for now: we find that Clover Configurator works well for mounting the EFI and also, the drive in the SATA bay seems to contain the EFI that needs editing, not the EFI on the NVMe SSD. If we had removed the SATA drive from the Mac Pro, the NVMe’s EFI would probably be the main drive initiating OpenCore at boot time.