now I'm reading comment threads about "but /nix violates the FHS!"
The FHS is a Linux standard. MacOS (however it's capitalized these days) is not Linux. Little things like the presence of /Applications, /Library, /System etc might offer the smallest nuggets of datapoint that, whatever MacoS reason for disallowing /nix, its noncompliance with a standard for a completely different OS that MacOS doesn't comply with either is probably not relevant
Meanwhile. I wonder if I can just reinstall from the curl|sh line, or will that cause conflicts with whichever parts of my old nix install it hasn't wiped out?
so far so good ... it told me how to remove the remnants of the pre-catalina nix install and it appears to have successfully installed a fresh nix