I didn't do the previous security update as I didn't feel confident with the NVIDIA web drivers situation. I just tried this new one and was able to boot after disabling the NVIDIA web drivers. Installing the new web drivers subsequently hosed everything as I feared.
I was able to recover, in about 30 minutes, using CCC from a clone I updated just before the process.
My /Volumes/EFI/EFI/CLOVER/kexts/10.12 folder contains:
NvidiaGraphicsFixup.kext
Lilu.kext
IntelMausiEthernet.kext
VoodooHDA.kext
With my current setup everything seems fine...startup is quick, graphics works well...except for a jumpy mouse that I have to deal with.
What's the trick to be able to update macOS? Do I have to use my current NVIDIA web driver and "edit the old NVDAStartupWeb.kext"?