I haven't had really hot heat yet this winter and last weekend I finally decided to do something about it.

At first, my car would get into operating range while driving around the city, but on the highway would almost dip below the coldest mark....right, so thermostat must be stuck open.

Replaced the thermostat and sure enough at highway speed, it stays at operating temperature now. I also had heat on the way home from the thermostat change....actually had to turn it off of max heat.

However, I parked the car, came back to it two days later, got into operating temperature, but no climate heat.

When I filled the coolant after changing the thermostat, I had the cap off until it started circulating and kept filling as needed as lil air bubbles came popping out; I'm 85% certain I have all the air out of the system. The reservoir is also at a good level between full and fill after I capped everything from the filling.

Some further information:
1) when I adjust the temperature control from hot to cold, it gets really damn cold (ac off)
2) the hoses going in and out of the heater core are both fairly hot
3) when I turn the fan off, count to 10, and turn to full blast the air feels warmer than it usually is, but it could be a subjective feeling...hands are getting cold in that 10 seconds, then adjust to the heat shortly after
4) the resevoir is sitting where the upper intercooling piping USED to go (beside the air intake) (I have a short route so I had to move the resevoir out of the way), the top of the tank is a little lower than the rad cap, but I read somewhere that it is supposed to be equal with or lower than the thermostat...is that true?
5) there are no coolant leaks, park the car, idle it, whatever, not a single drop of coolant

It seems there are four reasons for poor heat:
1) Thermostat stuck open
2) low coolant or air in system
3) clogged heater core
4) blend door not doing its job

So,
1) thermostat is fixed now
2) just filled, no leaks, kept circulating with cap off until there were no more air bubbles
3) both hoses are hot
4) when switching from hot to cold, it gets damn cold, and after I just changed the thermostat, I had really hot air blowing on the drive home (30mins) from the shop I did the work in

Based off of this information, what would be my next in line to test/fix? What additional checks can I do?

Thank you in advance.