So my '86 trans am runs so hot that I can't really drive it.
This is caused by the fact that the fan will not switch on.
Technically it works because when I'm running in diagnostics mode the blade starts to spin normally.
I have an original single fan setup and I have recently replaced the coolant temp sensor and a temp sensor on the driver side cylinder head.

I believe the fan is set to start at 222F which is a fairly high temperature. Previously my car rarely even heated up to that point. Now it goes up to 220F within 5 minutes of highway driving.
So there could be more issues than just the fan.. (ignition timing slightly off?) there is enough coolant in the car and I have checked that it circulates.

I have recently installed a 180 degree thermostat.

Apparently the fan switch is located in the cooler.

Could this problem be caused by excess air inside the cooler, for example? I had some trouble with adding coolant in the car.

Does the fan turn on when you engage the AC?

The fan switch is installed in the passenger side head, just forward of the oil dip-stick. This controls when the fan turns on in the single-fan cars. Make sure this is good and is hooked up. IF the fan turns on with the AC, then the relay should be good, but the wiring to the fan switch is most likely bad. If the fan does not turn on with the AC, then it coudl be a relay or wiring issue...or both. the fan itself is good because you report it turns in so it's a wiring issue somewhere. Just need to track it down.

Also, do you ahve the Air Dam on the front of the car? If that is on, you should never overheat on the highway, even if the fan doesn't work.

I replaced the new temp switch with the old, original one and now the fan starts at 222F like it should... somehow the new one wasn't functioning properly, I guess.

But I just noticed that coolant keeps disappearing from the coolant holder and there's a small leak on pavement right under the cooler...
Oh boy, yet another issue before I get to go cruising.

Is the cooler Finland's name for the Radiator? I have never heard it called that, and then I noticed you are in Finland. Also is the coolant holder the overflow bottle?
Is your cooler copper, aluminum, or plastic? Plastic radiators all seem, to fail at the seams about every 5 to 7 years.