Doing a new cat first is probably the most expensive item and the last thing I'd start with.
Always start with the lowest hanging fruit first. Generally these are the easiest and cheapest, then go from there.
Do yourself a favor and do a search on here. Type your code on here in the search box and just read as many threads as you can. This will give you a better understanding of what others have done.
Here's one example of a thread.
Scroll down to post 4, this member posted some very solid advice.

https://www.tundrasolutions.com/threads/p0430-up-down-stream.201738/?post_id=1959834#post-1959834

 

if the old motor was burning oil or ran really rich on that bank of cylinders, you may have damaged the catalytic converter. The secondary oxygen sensor is what checks the functionality of the catalytic converters (one on each bank typically). It wouldnt hurt to see what the upstream oxygen sensors are reading when the engine is warmed up to ensure it is not running lean or rich.

 

My Sequoia is unusual in that it has TRD Supercharger on the 4.7 but I have encountered that code recently. The code was reading catalytic efficiency almost every time I got under a load with the supercharger( even just a big hill feeling it load without downshifting) Turns out the mixture was being just enough richer that it was triggering the downstream O2 sensors. My problem was solved by adding small spacers on the sensors so for your issue I would definitely try new O2 sensors prior to the expense of new cats. My cats are the originals and therefore 24 years old and not replaced at least yet.

 

I have a 2003 Tundra. Perhaps get a new (free) reading at Advance Auto or AutoZone. Appears to be an O2 sensor.

It could also be an exhaust gasket at the muffler.