I don’t know how to solve your GRUB problem, but for this:
/boot gets a copy of all kernels and initrds needed for every one of your system generations. You can list your system generations on NixOS with:
sudo nix-env --list-generations -p /nix/var/nix/profiles/system
And clean up ones you no longer want to keep around with nix-env --delete-generations. (That won’t remove things from the Nix store until your next GC, but it will remove entries from your boot menu and prevent you from rolling back to the deleted generations.)