My issue was similar after buying a used 2015 JKU last year. Tried everything but the elephant in the room until we couldn't ignore it any longer: the heads/gaskets etc.
However, even after the head repair (which included a new water pump and thermostat), I was still overheating.
I have had quite the ride, so to speak, with thermostats. I'm convinced the MotoRad therms are junk. These therms are OEM and sold at AutoZone and Advance Auto (not sure about O'Reillys or NAPA), and the dealerships usually have a bunch on their shelves too.
I'd verify the part they installed was Genuine Mopar and look for the Chrysler stamp on the part. They may have installed a MotoRad at the dealership. If they did, get a swap for Mopar and test it out. Agree with air pockets too. I'd do it myself after leaving a mechanic, just so I know it's been done.
Good luck!
