Silkside

Vehicle: 2014 Silverado 1500 5.3l 120,000 miles So I've had intermediate issues for the last two months with low engine temperature (probably thermostat though could be temperature sensor) didn't think too much of it, fan would stay on full bore for a few mins after shut off and was planning to fix it when I got a second. Only started after I had the dealer throw in a new AC condenser (Fluids are fine). Now I have a bad catalytic converter code popping up and the truck is bucking like a damn horse as I go down the road a solid 100-200 rpm, more frequently when there's a load on the engine from hills. Did I just make a $40 problem a $1,400 problem by not letting the cat get to temperature? Any chance replacing the 4 - $100 O2 sensors will help or should I just slap the new cat on? (after thermostat is fixed obviously) Thanks in advance for the input! Jack