Head Gasket job done - overheating. Symptoms inside.

Details:

2008 Impreza 2.5i. 248K miles, healthy bottom end but the head gasket said it's time.

Heads were skimmed/resurfaced at a machine shop.
Felpro gasket kit was used for new head gaskets.

New Gates timing belt only. I wanted to go the full mile but money is ungodly tight (I'm quite experienced being a mechanic so I cover the labor).

I noticed the water pump is NOT OEM - I believe it was stamped GMB? it felt just fine turning.

Aisin THF-003 thermostat was used - said to be the only aftermarket alternative to OEM (I believe this is what I'm regretting).
 

Symptoms:
No codes
Overheats in just a couple minutes with a test drive
Little to no heat at idle. When held around 2-3K it gets warmish. Coolant bleeding funnel was used.
Coolant is Prestone Generic 50/50 - not optimal I know but it doesn't have to last very long in 2025 its a beater at this point.
Upper radiator hose - hot
Lower radiator hose - cool/cold
Upper heater core hose - hot
Lower heater core hose - cool/maybe luke warm?
 

My thoughts:
It's like the thermostat isn't opening? Everything else seems fine - it's clearly getting hot. I'm guessing there is little to no heat because that hot coolant going into the heater core cannot circulate through the radiator. No circulation = overheating of the engine. Is this going to be a 'I should've used an OEM thermostat thing'? I removed the thermostat housing outlet and the coolant coming out of the thermostat weep hole was hot. Is the replacement thermostat stuck shut?

Thoughts/suggestions please - no need to hold back. This thing is killing me I've been down 2 weeks between waiting on holiday shipping times and the machine shop. I can answer any question you may have. Thank you for reading - save my ass please lol
 

Update:

I couldn't get to it yesterday but today - despite it raining all friggin day - drained the water, back flushed the heater core and basically the entire cooling system with the hose, aftermarket thermostat back in, torqued correctly, filled with part water part coolant and...I was not nice to the car about it. I revved the fuck out of it, gave it hell, let it idle for 20 minutes.

Went on about a 20 mile drive and...heat seems to work, not as well as it used to but when not at idle its hot. I never saw a red overheat light popup but get this shit. All year I suspected something wrong with the starter as it sounded...tired. It acts like a dead battery trying to start (its a year old, Bosch brand) alternator is good - had a orileys test it and my starter is on it's last few days of life lmao. I can't catch a break man