After your car's been warmed up with the heater on at least 20 minutes, grab the top radiator hose. Pretty warm/hot? Now feel the heater hoses near the firewall. Both nearly as hot? It's your blend door. Heater hoses only coolish/warm? It's a clogged heater core, or if your car has one, a water control valve.
I too think it is the thermostat. You need to take it out and test in in a pan of water on the hob. Suspend it on a piece of string. It should start off closed then open fully just before the water boils.
Sure sounds like a Thermostat. Classic symptoms.
I think you are wrong about knowing it isn't the thermostat because all symptoms point to that.
But, water pump, bad hose, electric fan not running are other possibilities.
thermostat stuck open , they are designed to fail in the open position
Just low on water can do that as theirs not enough to push through heater, just check.
thermostat could be stuck open.
heater core, blower fan
I know it is not the thermostat. What could it be?
