I started using Nix around 3 years ago, and I actually didn't encounter this issue. The last issue was when Catalina made root read only, there is some hacking to keep /nix in the root.

Edit: after searching about it, looks like this was the same issue with /nix in the root, and disabling SIP was one solution at the time. Currently the solution appears to be creating a separate volume and using /etc/fstab and /etc/synthetic.conf to set it up[1]. The installation script does it automatically.

https://nixos.org/manual/nix/stable/installation/installing-...