A big temperature gradient from top to bottom of the rad like that seems to suggest slow flow though the rad (the water has plenty of time in the rad to cool before it is pumped back into the block).
If you've had nice, clean coolant for ages it seems less likely to be blocked and more like a feeble or not working water pump, or the thermostat is not opening fully.
Some water pumps have a plastic impeller that can detach from the shaft, some have a metal impeller that can detach from the shaft.
The thermostat is much easier to check / change than the water pump.
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