Troubleshooting a P0420 code

Hi All- My ride is an 09 with 225K miles. Its been a great car, but showing its age a little. I am not much of a mechanic, so bear with me.

I have a BAFX ODB2 reader and use the Android Torque app (paid version). It is giving me a P0420 code- catalyst system efficiency below threshold (Bank 1).

After researching, I learned the Prius doesn't have an upstream O2 sensor, rather it has an air fuel ratio sensor. This makes sense since I couldn't get a reading in the Torque app from the upstream sensor. I am trying to determine if the AFR sensor needs to be replaced, but I am not sure which sensor to select in the Torque app. Then once I have the sensor selected, I need to understand what the volt reading(s) should be. I did try the AFR(m) display within the Torque app, but I am uncertain if that is the correct sensor and I don't recall the reading it was giving me. Anyone have experience with finding the AFR sensor in the app and how to read it? If I can determine the AFR sensor is faulty, I'd like to replace that myself.

At a minimum, I'm thinking my catalytic converter needs to be replaced. The downstream O2 sensor is giving me a sine wave reading. From my research, it should be a relatively steady volt reading and the sine wave suggests the catalytic converter is not functioning properly.

I plan to take my Prius to my mechanic to replace the cat unless I later conclude it isn't faulty. I'm not subject to emissions test. Given the mileage on my Prius, should I go with a direct or universal fit cat? I'm planning on selling this car during the next 18 months.

tl/dr - Which display do I select in Torque to read the air fuel ratio sensor and what is the acceptable volt range for this sensor? Direct or universal replacement cat?