A very high level in the coolant reservoir is not completely reassuring, since boiling of coolant happens inside the engine. That increases the volume inside a closed chamber, which pushes fluid back into the coolant reservoir. After the engine cools, if the cap is working correctly, coolant siphons back, but trapped air often remains inside engine passages. Just because you have little air inside the coolant reservoir doesn’t mean you have properly burped the system. That needs to be done first.
If you are just going to replace more parts at this stage without further diagnostics, I would not replace the radiator again. Rather, as I said above, I would test for combustion gasses inside the coolant system. You can buy such a tester at any parts store for about $50, or you could ask the mechanic who did the head gasket to do it for you. If you still have products of combustion there, your head gasket repair failed. If you want to replace a part without more diagnostics, then a new thermostat would be a lot less expensive than a radiator.