I get good heat and the gauge doesn't fluctuate whatsoever so I don't think it's a matrix problem. I actually think it's stuck northbound of the thermostat. Which is why I'm surprised that when I bled the system with screw right by the 'stat and topped up the coolant again, it just drunk it.
well that sounds like you have coolant flow!
are you sure the thermostat is fitted correctly and not somehow stuck shut?
dont forget the new stat is lot hotter than the old one so the coolant will get quite hot before it opens and allows coolant flow!
run the engine with heater on full heat and full speed and check the pipes getting hot etc to and from the matrix
watch the guage and as long as it doesnt fluctuate then it should be ok
feel the top hose for hot coolant flow in to the radiator... if its gone through a couple of warm up cycles then it could have
bleed most of the air out