08 Toyota Prius P0420 code and maybe bad cat converter?

I have a 2008 Toyota Prius. Have been registering the P0420 code. This happened last year, so I replaced both the upstream Air/Fuel sensor and the downstream o2 sensor. Change my spark plugs and also installed a new MAF sensor, change the air filter. I reset the code and still got the p0420 code. So I installed a downstream o2 sensor extender. And reset the code and it stayed off. But it came back on recently. So I replaced the downstream o2 sensor. The old one was really black carbon covered, as was the extender right through the hole. So I removed the extender and installed the o2 sensor without it. Did no good, as the Po420 code keeps coming back.

So I decided to look at the spark plugs (Denso Iridium) and they were all kind of evenly whitish. When I saw the o2 sensor with the black carbon, I figured the engine was running too rich. But if that were the case, wouldn’t the plugs also have some black carbon deposits? Without doing the heat testing, seems possible my cat is bad? I am not getting any rotten egg smell, or gas smell, or leaky exhaust smell, or burnt smell, and my engine seems to be running smoothly. And I don’t see any holes in the converter or pipes! I should also mention that I had the same problem with my 2004 Prius! Seems this freaking P0240 code is a pain in the ass for these cars! I live in Syracuse NY, so good luck with inspections! Luckily I am not due yet.