One thing I haven't mentioned that doesn't run properly under Big Sur is Rod Smith's
rEFInd Boot Manager for selecting your OS at startup. It doesn't run properly with Catalina as well.
Prior to Catalina it would see my Mac system (Sierra, Mojave, etc), my Windows system (Win 7, Win 10), and any implementations of Linux I had installed (Linux Mint, Ubuntu).
After I installed Catalina,
it doesn't see the Mac OS any more. Correction -- It sees the MacOS system, but once I boot into it, the boot partition is modified so that rEFInd doesn't work any more. I then have to re-install rEFInd.
I think it is because the Boot Rom has been updated.
So, now, if I want to select Windows or Linux, I let it boot as normal and select the OS from rEFInd.
If I want to select MacOS, I have to hold the Option key down and choose the one I want.
At least it works.