Side note: I hate launchd, and how bad MacOS things in general handle symlinks. I ve never managed to restart nix-daemon after an upgrade, or after I edited /etc/nix/nix.conf. If I `launchctl stop system/org.nixos.nix-daemon`, it wouldn't stop. If I killed it and `launchctl start system/org.nixos.nix-daemon`, it wouldn't start. If I tried to `launchctl unload /Library/LaunchDaemons/org.nixos.nix-daemon.plist` it would say it didn't know any service '/nix/store/...'.
Now I copied the file there instead of symlinking to the latest nix, and changed the absolute reference in the file to /nix/var/nix/profile/nix/... instead. Now I can unload and load it as I should have been able to all along.