It is normal for coolant level to go down after refilling the system. I always have to top it off after flushing and then driving it for a short trip. Many areas for air to hold itself in until the system comes back up to hot and pressurized as well as driving. You must also turn the heater on and let that run for about four minutes to get that coolant circulating and clearing through the system.

Taking the cap off the reservoir after the car has warmed some will immediately depressurize the entire system; You shouldn't be hearing a hissing sound for any length of time. If you do something is plugged or sucking air trying to normalize to the atmosphere. Find the hiss and you will find one problem.

The plastic side tanks of the radiator is often the first place a radiator will start to leak. i tried to make a repair to just such a leak a long while back and it didn't work.
Buying a used radiator is asking for trouble

Have you checked the transmission fluid level? It too flows through the radiator.

Are you using distilled water and GO5 anti freeze 50/50 mix?

Most definitely remove the serpentine belt and check all the bearings for wear and the water pump particularly. Be advised the water pump has multiply mounting screw lengths; Pay close attention when removing and installing.

Does the thermostat appear to work correctly. If it is original you should replace it.

Are you using a chemical testing kit to test for blown head gasket? Good luck with the test! It is the best way to go.