I spent an hour bleeding the radiator and still the cabin air is cold

You should probably check your heater water pump and heater control valve (3-way switch valve) and I believe you have a coolant bleed valve on a 2008 Prius. Follow OEM procedures and independently verify that coolant is moving where is suppose to. Just feel the external hose temperatures to verify flow.

 

Did the coolant loop ever stop bubbling? If not it isn't full.

The capacity of the entire loop is 9.1 quarts. Normally on a drain and refill only about 6 quarts come out, but perhaps you drained more because of the water pump/thermostat/radiator? Did you measure the amount that came out?

Car was losing coolant, was there an obvious leak? If not there could be a blown head gasket and the missing coolant is going out the tail pipe, and exhaust fumes may comprise some of the bubbles that are being observed. That isn't a common failure mode on this generation, but it does happen. Also the OP notes 320K miles, and at that age what is "normal" for cars under 200K may not always apply. The 2nd generation head gasket typically lasts over 250K miles, but I don't think anybody here expects one to last forever.