That P0420 code can pop on for a number of reasons. Very rarely does it actually involve anything wrong with the catalytic converter or even the O2 sensors. Unfortunately, lots of techs only look up the code and replace parts instead of properly diagnosing the problem - that's usually why you see lots of posts of upset owners that have replaced a number of cats + O2 sensors and still have the code get set.
That said - if the car is running well otherwise, it very well be due to the initial threshold set in the ECM. An ECM reflash will help in that case (also the same fix for the excessive sulphur -dioxide issue with some 9th gen Corollas). Here is the catch - only the Denso ECM can be reflashed, the 9th gen Corollas with a Delco ECM cannot get flashed. Those cases, the ECM is just replaced.
Looks like you are also outside of the "Toyota specific major emissions components warranty" of 96 months / 80K miles. Reflash can be done with just the diagnostic charge - usually around $85-$95 - way less than a cat or O2 sensor replacement. If you are unlucky and have the Delco ECM - you'll be hit with a pretty steep bill for the ECM - street prices go anywhere from $1000 to $2000 easily. Some opted to get used ones from a salvage yard to save on costs, if you can find still find them.
Another option - trick the ECM. Look online for the sparkplug defouler mod on the downstream O2 sensor. Basically it moves the downstream O2 sensor just outside the bulk of the exhaust stream and "tricks" the ECM that the resulting waveform is within preset thresholds. Doesn't work for every single case, but due to the very low cost (couple of bucks) - usually the next thing some owners try if they are unsuccessful in getting the ECM flashed.
Since you've started a paper trail on the replaced O2 sensors and catalytic converters - might be able to show that to the dealership and have them consider you for a "good-faith" warranty work. Varies from dealership to dealership - some might cover 100% of the costs, even if the car is outside the warranty period - some might cover some portion of the repair. You won't know until you ask. Good luck.