Assumtion The car was not overheating prior to waterpump, thermostat, and flush..Just blocked heater core..
Seems most people (as well as I) believe that all these GM products was/is caused by the "improved" new dexacool antifreeze and previously leak stop put in on the assembly line...Turns coolant into Alabama red Mud after 5ish years..
Only two items I have found that cuts it loose at all..Best is Thermo cure by evapo-rust and a few days of driving..Man it stinks!! And 5 to 10 flushes to get most of the junk out. (Red clay)..of course it can't be freezing outside...
In eight minutes, the little 3/8 line to coolent tank, one of the heater hose is no longer comfortable to hold IR gun say 141 degrees, same as water pump.
After 10 min, thermostat open, scan gauge says 190...driverside radiator tank reads 160, same for one heater core, same as 3/8 coolent line...Passenger tank around 110...(note, this is outer surface of lines/tank with ir gun)..above 140 degrees it is uncomfortable to hold.
One can not see the waterpump drive coupling, nor impeller without voiding warranty.
I think you have no water pump flow...drive coupler, no/slipping impeller..
Blockage or stuck thermostat would not stop all flow...take it back to the shop that I'm sure you paid good money too..My two cents..kev