We need to get this fixed. Winter is close. LOL
As far as the heater, it really is straight forward. Hot engine coolant passes through the heater core and when heat is requested, the blend air door opens, and the fan blows across the heater core sending hot air into the vehicle.
You have already checked the blend air door actuator and it is functional. So, the next thing is we need to confirm there is hot coolant passing through the heater core.
Do this. Start the engine, turn the heater on high, and allow the engine to reach operating temperature. Once it does, turn the engine off. Open the hood and feel both heater core hoses. One should be very hot and the other will just be a slight bit cooler. One is the supply (very hot) and one is the return. If you find one is hot and the other really isn't, we have a plugged heater core.
Other than that, it could be low on coolant, have an air block in the system from recent work, or the engine may not be fully reaching operating temperature.
Let me know what you find. Also, here is a link you may find of interest:
Sunday, October 29th, 2023 AT 7:21 PM