I'm stuck with a Gen 3 Prius and I'm panicking

I have no intuition for car repair in the US, because it's wildly different from back home.

Back home for example stabilizer bars/sway bars aren't consumables, the bearings inside them are simply swapped and regreased at regular intervals, but here if people won't do that which understandably could be a liability issue but then it's upsold into a whole suspension assembly project. Is it justified? Is it because the roads are different? Who knows unless you know? I didn't think the suspension needed to be replaced on my last car, from a lifetime of diligently maintaining my car where people are driving on horrible roads with no issues. All I know is guy gets pissed if I do a diagnostic anywhere else. This man had me change out the entire suspension when my gut was saying this makes no sense when the stabilizer bar was the only thing that seemed like needed maintenance. But then I've never driven in a country with the highway speeds people put their cars through here, and only ever owned/maintained small city cars and an occasional sedan. I was referred to him by family, who I'm realizing are equally clueless and just throw money at problems and hope it goes away.

I got suckered into buying this car by my local mechanic who looked it over and said it was a good deal after I said I was comfortable handling the eventual battery swap if needed by myself. He then immediately started quoting me thousands of dollars in repairs after doing another diagnostic a month later. I had budgeted for problems to pop up but the laundry list of things he quoted me was crazy.

How do you trust anyone in this environment?

I went through a months long process of trying to convince every other snake in this business that I needed a water pump change, with people swearing up and down that it's a head gasket issue, that the water pumps are electric so they don't fail, and a "former Toyota technician" saying he's never seen a head gasket issue in these cars but he needs to do a 200 dollar diagnostic to redo the test for coolant contamination that I got somewhere else and refusing to work with a sourced part because he wouldn't do a relative compression test, being quoted 600 to 800 dollars to swap a water pump I provided and finally landing on a guy who acts like a real human being and answers questions an hours drive away who was willing to do it for 150 dollars. Even he has some rich partner an hour in the other direction running interference from afar trying to tell him to convince me to get the head gasket replaced.

I don't even trust these people wanting to keep a car I need every day for a simple check on compression, they want it the whole day, when back home most Autels and a techstream setup from what I can see could do a relative compression check for that in a second even on hybrids. The constant unfettered visible and obvious greed in this industry is amazing to me. My water pump failed in a way that showed up as code and only then did everyone suddenly feel like it was the cheapo part all along. Like it wasn't obvious from the overflow spraying coolant everywhere. Immediately they were all like yeah we replace them all the time, look at these ones I swapped out 500 bucks will get it done, I can do this for you. Just gross.

I feel like if I don't work on my car myself these straight up predators will always have their way. I have the 0420 code now and I just need to actually figure out if the water pump failure actually caused damage to the head gasket or if it's the O2 sensors or the cat. I never had to do any more than the most basic maintenance on the car and COVID forced me to move to this country with no long term plan because my industry practically shut down across the world while I was here so I had to switch to something else. I have no access to the tools and equipment that would have been at my disposal where this wouldn't be the problem anyway.

How do people survive here? It's insane to me. How are there not community garages for people to learn how to work on their cars when everything is car dependent and you're really likely to get fleeced otherwise?

I don't have the liquidity to deal with huge surprise expenses, and knowing this thing is throwing code and I can't get basic advice from my mechanic is driving me up the wall. I have a good Bluetooth OBD module, and the torque app, hybrid assistant, dr. Prius etc. I knew in theory that you'd chart the O2 sensors against fuel trim, accelerate for rich and pump the brakes to check how it reacts to a lean mix but the Prius has a brake booster, so that wouldn't work? I don't think I'm losing coolant that could cause the CAT to get messed up, and there's no weird smoke, rough idling or some such. I don't really feel a loss of power but I don't drive very fast anyway. I had one misfire cold starting the engine during a snap freeze when it was parked outside months ago that I haven't had since.

I'm kinda lost and don't want to miss whatever the right workflow is for car culture here because I can't even communicate about problems it's wildly different. Forum culture usually dictates you know what you're talking about somehow already, otherwise half the advice is conflicting and the rest is you're looking in the wrong place. I hope this place is different.

I want to compile a list of tools and tutorials to cover most basic gen 3 care and maybe even things like head gasket repairs, because the 3000+ dollars I get quoted out the gate for the head gasket might as well go down the drain the moment the car gets hit or pops up with another issue that's completely unrelated or the actual root cause. It's what everyone says you have to do to save money or have reliable repairs.

I'm glad I found this forum, I wish this was less stressful. I'm sorry for my tone. I've already gone through a nightmare ordeal with the healthcare system here so this tone is a lot of projected frustration from that too. I'd rather have the means to deal with this myself vs leaving it entirely up to people I don't know and have no real way of trusting.

Assume I know nothing, where do I start?