Yeah, that's a long-running and pretty much unfixable issue as macOS updates overwrite /etc/zshrc. AFAIK, the determinate systems installer does somewhat resolve this as you can just run it again and it will fix the issue.
The default installer is not idempotent yet (and my work on trying to resolve that has stalled, unfortunately, see https://github.com/NixOS/nix/pull/7603), so trying to run it again to fix this issue will result in errors.
Yeah, I generally uninstall (with the remaining nix-installer binary on the nix volume) and then reinstall. It would be great if the installer were idempotent and I could just re-run the installer!
The default installer is not idempotent yet (and my work on trying to resolve that has stalled, unfortunately, see https://github.com/NixOS/nix/pull/7603), so trying to run it again to fix this issue will result in errors.