I was annoyed at having to type :lf /etc/nixos all the time, so I wrote a precursor to this & improved on it for a while; I think it’s pretty good now.
You can load it in a file like ~/repl.nix and provide all the needed arguments (isUrl is mandatory if it’s not a file path, but a flake URL), and then call nix repl ~/repl.nix (you could make an alias, I guess—I just use my shell’s history autocomplete)