As far as I can see you don't define any way to start Tor in nix but that is how nix normally passes along the tor config.
ExecStart=/nix/store/<hash1>-tor-0.4.8.14/bin/tor -f /nix/store/<hash2>-torrc
ExecStart=${pkgs.tor}/bin/tor -f ${localTorrcDefinition}
You could define a file in etc that is your torrc, then point your service manager to use that as the -f argument for Tor.
Another way would be to create a small derivation that copies the default tor derivation, overwrites the etc folder and then use that as your system Tor. (See runCommand)