red triangle of death—help?

Howdy,
so my 2005 prius is coming up on 300k miles on the engine. It's been a real trooper. But suddenly this summer, it gave me the dreaded red triangle. I didn't have the time or energy to troubleshoot it or the money to pay someone else to, so I let it sit for a few months, and now I've been trying to figure out the problem.

I got a basic OBD scanner, and the codes I'm getting are: P0037, P0420, P1121, P1150, P1116, P3190, P0301.

I also took it in for a basic oil change and fluid top-up two weeks ago to rule out any issues from sitting too long (it was due for the oil change anyway).

Known issues: the engine has been burning oil and needing regular top-ups for the last couple years, which seems normal for the age of this engine.

I did a DIY catalytic converter replacement in early 2021, and it has been working fine since. Non-OEM parts for both the cat and the O2 sensor, which was cut when the cat was stolen. The replacement was not my best ever welding job but has held up ok.

From my research, and here's where I could really use some help because to be frank I know just a little more than shit about cars, the codes I'm getting fall into two main buckets:

A: Emissions/cat, P0037/P0420

B: coolant system, P1121/P1150/P1116

I've been focusing so far on B, the coolant system, because this seems most likely the cause of the red triangle and potential cause of serious damage. In my research on the forum this combination of codes seemed likely to be the result of air bubbles in the coolant system somewhere. I followed a youtube tutorial on 'burping' the coolant system, but honestly it doesn't seem like there were/are any air bubbles. As we speak I'm waiting for the engine to cool so I can see if it needs any more fluid, but from what I can tell there are no leaks, no air bubbles, the temperature is correct (OBD sensor read it as 85 C once engine heated up), the cabin heater is working and I can't hear any glopping sounds behind the dash. Seems like fluid is circulating since it's heating and nothing is overheating but idk how to check for a clog anywhere.

So, what can I try next? Replace the 3-way valve?

Could my A cluster problems be causing the red triangle, and will they cause further damage to drive on? If I have an exhaust leak, how can I tell where it is? Research seems to indicate I should address A cluster problems before B, but I'm not sure why.

 

If I take it to an autozone or jiffylube, will their scanners give enough detail? or do I need to take it to a dealership you think?