It's a pain chasing such faults.
If your purchased pump had a metal impeller I wouldn't expect there to be an issue. Specifying Volvo only is fine if you've got money tree's in the back garden.
Your hot top, cold bottom hose is classic trapped air but are you also confident that the lower portions of the radiator are clear so that flow is unrestricted other than when the thermostat is closed?
Borrowing/buying an infrared thermometer would be good to take a snapshot of individual cooling system component temperatures which could help identify an issue.
One method of removing air is to elevate either the expansion tank or the corner of the car where it's situated, ie jack under front o/s of the car. Then run the car with expansion cap off, engine running heater on hot, watching for bursts/bubbles of air in the header tank, coolant at the ready to top up with.
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