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The cool to the touch lower hose is a sign of no flow through the radiator. Could be either the thermostat isn't opening, or you've had a water pump failure. If you've driven it for some time since the last cooling system work without any issues, and now it's started the overheating then not likely to be an air pocket.
Did the new water pump have a metal or plastic/composite impeller? The plastic impeller water pumps have long been a weak spot in the BMW cooling system. BMW changed to a composite impeller which is much higher glass fill than the original, but even they have a shorter life than the metal impellers. The failure mode is the impeller cracks where it's molded onto the shaft, comes loose, and stops pumping.
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