Deceleration Fuel Cut Off Inputs/Problems?

I've got some questions maybe someone can help me with... I'm working on a 2002 Toyota Sienna V6 3.0 DOHC 1MZFE with an intermittent P0420 cat efficiency code. Was brought to me with a #2 cylinder misfire and quickly diagnosed as mechanical problem with no compression and bypassing rings to crankcase. I replaced the engine and the car runs great with no driveability issues. However I did advise about the possibility of cat damage depending on how long its been driven with a misfire. So to get to the point... it pops a P0420 every couple of months or so. Before I throw the diagnosis of "it needs a cat" I'm trying to do my due diligence. I am not seeing the downstream O2 sensor oscillations that I would normally see to condemn the cat. DS O2 reacts to lean and rich conditions as expected and really doesn't float from rich to lean to rich much... mostly fairly steady. My upstream AF ratio sensors don't seem to have quite as much fluctuation in voltage as a 2016 Corolla that I was using to just check what I should see. However the AF sensors get a tad above 4 volts in decel fuel cut and 2.5-3 volts at WOT so there is normal movement there. My issue is the car doesn't go into decel fuel cut unless you are at high speed (60+) or high rpms (1800-2000). It does not enter decel fuel cut at all during lower speed conditions. Service data says it should return to fueling at 1200 rpms. When you can actually get it to go into the decel fuel cut and then it returns to fueling at 1800-2000 rpms, the AF sensors go from the 4+ volts back to 3.1 to 3.4 or so and bounce around a bit. It is also at this time that the downstream O2 sensor kinda goes a bit wonky. I'm thinking that this might possibly be what the ecm is interpretting as a cat efficiency problem? So I'm wondering if that sounds at all plausible or am I chasing my tail? I'm not exactly sure how to diagnose a decel fuel cut off issue. I'm guessing its an input problem but what and where to look. Engine seemed to run a tad cool but not by much at 174 degrees when specs call for 176-183. I know coolant temp can affect decel fuel cut but I don't want to go throwing a thermostat and ECT sensor at it over 2 degrees. Other possibility maybe a throttle position issue? Crank relearn which I didn't see an option for on my scan tool. I don't want to guess and say it needs a cat which is obviously expensive... but the time trying to diagnose the decel fuel cut off can get expensive too and I might even be barking up the wrong tree. Any info or ideas would be great!