Which mini-guide are you referring to (simply post a URL link)? If you're referring to the Radeon Boost mini-guide, we should remove the boost kext or SSDT when installing a macOS update. We should also remove them when troubleshooting (i.e. process of elimination) to see if that makes any difference.
The boot argument agdpmod=pikera is already in the NVRAM section of OpenCore Configurator. But it's good to check and make sure.
I would definitely not recommend installing Big Sur on top of Catalina. It's best to install it on a spare SSD. At this time I cannot provide any support for Big Sur, but you're welcome to experiment on your own. It is a refreshing experience.