2008 Prius with heating issues and intermittent Red Triangle indicating overheating

I have an 08 Prius with 160k miles. This winter, I started having serious cabin heating issues. The cabin heat basically stopped working but would occasionally work about 20-25 minutes into driving, and this was often including "warming up" in my driveway for 10-15 mins before driving. I had a check engine light on for awhile indicating, if I remember correctly, that it was probably the three way valve or inverter. One day on the way home from work, I got a red triangle of death and the car basically slowly lost power so I pulled on the side of the highway and the car would turn on but not move. Eventually I solved this issue I believe by disconnecting the wire for the inverter (I think?) above the fuse box and replugging it back in. I got the car working enough to get home and decided it was time to try and fix whatever was going on.

For what it's worth, I previously had an issue like this years ago that happened on a trip back from Buffalo (8 hour drive for me) where it seemed the car was overheating and shutting down. I had to spend that whole trip back by driving a little bit, stopping somewhere and letting the car cool off, driving a bit more etc. til I got home. We replaced the three way coolant valve if I remember correctly and the issue went away but I often wonder what damage I did to my car on that drive home.

So finally I decide I'm going to fix this via process of elimination if I can. I bought an OEM or OEM equivalent part (Aisin) for the Inverter pump and the three way valve. I installed both of them, no problem. Drained the coolant when I did it and refilled the coolant with the airlift system to make sure air bubbles wouldn't be an issue. Still, no heat in the car even when letting it sit on for awhile with the heat going. Next step was the thermostat, I removed it and put a new one in. The old one actually was working, confirmed with a boiling water test, so it definitely wasn't the issue. Then I tried the heat again, of course nothing. I had a code p1150 also at this point, which I read could be the coolant temp sensor, so I replaced that (easy job, though I had to re do the coolant again). Once again, nothing as far as heat.

So today I decided to try and take the car to work just to see how it would do. First of all, heat DEFINITELY isn't working at all in the cabin. I drive about 15 mins and start getting an intermittent red triangle accompanied by the red thermostat looking signal on the screen to go with it. From what I've read, this means the system is probably overheating? So now I'm at a loss. Is it a head gasket issue? Is my coolant system clogged somehow? I really have no idea and I'm hesitant to sink more money into troubleshooting, I want to find the definite cause of it honestly. My wife has an 04 Prius with 130k miles we were going to sell (It has some weird quirks like a slightly crooked steering wheel, missing the radio dials, cruise control doesn't work etc.) but the heating in it works PERFECTLY and quickly, the cabin heats up within a few minutes. It needs a new catalytic converter and at this point, I'm considering just swapping the converter and radio from mine into hers and then selling mine but if it's a fixable issue, I'd much rather keep mine tbh. lol. Anyways, any insight or tips on this issue is very welcome, I am desperate at this point lol

 

I guess I'd be surprised because I re filled it using the Airlift vacuum system, which refills it to full as far as I know. It definitely did for my inverter system because it brought it perfectly to the full line on the reservoir.

The three way valve was replaced and definitely installed properly. The coolant was drained and refilled. How would I go about finding a clog?