Just so we are on the same page:All sorts of additives and anti-freeze have been used. The radiator seems to have a good flow. That made me suggest removing the pump and examine the passages, since the radiator was removed.
If the "suction port" is clogged or the wrong gasket is used, all suction effect will be on the "radiator port". That together with choked passages inside the block/head could give an abnormal underpressure on the suction side of the pump (= collapsed lower radiator hose), just like a choked radiator would do.
It is a theory that may be wrong, but I think it is better to examine things as far as possible to try to find an explanation. You may also find other problems to fix while you are at it.
But, without a functioning thermostat, the system is not working as intended.
Passage seen in screenshots from:
View attachment 75093
View attachment 75094
The only suction in the entire system is on the suction side of the pump - ie, through that lower radiator hose.
The rest of the system is under pressure.
The pump sucks water from the lower radiator and Pushes it through the water jacket then through the upper hose back to the radiator.