I hit mine with some penetrating oil and let it soak in an hour or so before starting. That seemed to help on the right side one. It broke loose with little effort, once I finally got the socket in place and the two wrenches lined up on it to twist. That part took a lot of effort, but the lube definitely help to break it loose.
The left side was seemingly unaffected by the penetrating oil, which is not surprising since I couldn’t really see it well enough to make sure I got the oil properly applied. Combine that with the fact that I couldn’t get my hand up in there (or down in there either) well enough to get a good grip, and breaking that side loose took exponentially more effort.
I was going to suggest this - good idea that you did it. Especially since those upstream sensors thread into cast iron manifolds and would be rusty around the threads.
Probably soaking with more would have helped that left side one. Sometimes getting a cotton swab soaked with it and applying it liberally on the top of the theads would have encouraged the oil so soak around the threads.
Generally O2 sensors start gettting a bit sluggish in their old age - probably more so as there's deposits building up on the sensors' tip.
For the cats - if you here rattling noises at idle - one quick way to test (cooled down converter) is to rap the body with a mallet (or your fist, wrap in a towel) and if you hear rattling - that converter honeycomb is breaking up and due for replacement.
Since the 'Hauler hasn't thrown any codes, the cats are probably still doing their job and within the range of tolerance to check out as okay.
Odds are you'll probably have better idle quality and the MPG might improve a bit as well
poor maintenance (fouled plugs, rich mixtures) often cause overheated and failed cats. Or someone putting leaded fuel into the vehicle