i had similar symptoms which everyone and his dog assured me was a failing head gasket, but turned out to be air in the system.
Some cars are difficult to refill completely by conventional means, but there are vacuum fill devices available to help fill these difficult cases I don't know if your car is such a case..
I don't have one, so now I purge air by boiling the coolant (fan disabled) into a funnel wedged into the rad filler port. This is a bit alarming, but I think its safer than running the car with an air pocket.
IF you have an air pocket, it will probably clear itself eventually, (mine did) but there seems to me to be a risk of damage if it moves to an especially hot part of the engine, such as the head near the exhaust valves.
As well as the exhaust-gas-in-the-rad test, you could try getting the cooling system pressure tested.
You can also test (look up "blotter spot test"for DIY) for traces of ethylene glycol in the oil, though this is usually only done for fleets.
Severe oil contamination will be obvious and you've said you don't have that.
As far as I know there is no test (apart from visible steam) for traces of coolant in the exhaust.
Edited by edlithgow on 31/01/2023 at 01:58
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