I just wanted to report that I installed the new 10.15.7 security update
problem-free, and tested AppleTV DRM, VideoProc, and Geekbench on
my iMacPro1,1 SMBIOS on an HP8300 with an RX560. New Build is 19H15.
Also, I was reading the Dortania guide today, and noticed it calls for
installing SSDT-USBX for SMBIOS iMacPro1,1, so I installed it. It seems
that the kUSB power levels that I can see in IORegistry match those in
the SSDT. Still testing and working on the Big Sur update...
6:46PM update
Big Sur update went smoothly. New build is 11.0.1 Beta (20B5022a).