It sounds like your engine is fine. You even said the transmission just doesn’t shift up. Changing the throttle body, the air filter or even the engine isn’t going to make the transmission shift properly. Change the engine? Are you kidding. For sure it’s because you are using the wrong brand of tires! Not really. You said you changed the transmission TWICE? I don’t believe it. Maybe you changed the transmission fluid twice although I doubt that because there’s a good chance THAT would help. When the car is supposed to shift all sorts of little valves have to slide around shifting hydraulic pressure from here to there and if they are all sludged up, they get stuck and the transmission can’t shift. Unfortunately the Fit doesn’t have a transmission pan you can take off or a transmission fluid filter, so at this point it’s going to be hard to get the sludge out, if that’s the problem. You could drain and refill it with clean Honda ATF, but that only changes about 3 quarts. Maybe a couple of changes about a week apart would help clean things up.