You can use synthetic.conf(5) to create a virtual empty directory at / instead of a symlink. That can be your mount point. To do this, you just need to add the following entry:
sudo vim /etc/synthetic.conf
add a new file with the following content:
echo -e 'nix\t/System/Volumes/Data/nix’ | sudo tee -a /etc/synthetic.conf
NOTE: Use \t (tab) to make /etc/synthetic.conf work on osx. Note 2: /System/Volumes/Data/nix must be a case sensitive filesystem
This will tell osx to mount /nix with the external disk directory nix. In our case, we used an external disk, but the directory could be at your home.
sudo chown -R <username> /Volumes/<username> /nix/ export NIX_IGNORE_SYMLINK_STORE=1 curl https://nixos.org/nix/install | sh