Found the OBD2 port! For the curious, on the 2016 it is not on the firewall but on the underside of the dashboard, adjacent to the hood release. Also the Carista app works well. My sanity is restored with only a single beep in reverse!
I don't know about 2016, but the 2018 prius OBD2 port is located underneath and to the LEFT of the steering column, NOT the right, or near the brake. Above the brake is where the OBD2 port was located for gen 3. The 2018 gen 4 has a plastic cover over that location, so if it is the same for the 2016 Prius, I can see why annexedwa would get confused... ...I did exactly the same thing.