But there is this persistent problem (Three Years Now)
Rear heater core has been blocked off.
Obvious question :
Has the rear heater been disconnected for the same amount of time of your issue?.
If so reconnect it.
Considering you state you’ve replaced MOST of the components in your cooling system ( you never mentioned the cap or hoses )
I might suggest you try the cap if you haven’t replaced it already.
Also check for a hose(s) collapsing causing a flow restriction.
You may have a partial coolant blockage inside the engine block.