bathroom junction box has more wires than expected
You are almost right

The solo black wire is indeed the switched hot for this circuit and the junction of all white wires is indeed your neutral.  However, you shouldn't tap ground at the box screw, but in the wirenut with the bare wires in it (in the back of the box) -- the box ground screw isn't intended for two wires at once.

P.S. the other junction (one white and rest black) is where the always-hot is -- the white wire in it carries the always-hot to the switch in an old-style switch loop configuration.