Poor gas mileage?

My car has been experiencing poor gas mileage pretty much since I’ve got it. I got it used off Facebook marketplace which I know is probably already a red flag but it’s all I could get. It has no check engine lights, and no lights on the dashboard besides a tire pressure light, which is just faulty. It drives perfectly fine transmission shifts perfect sounds perfect. Gets regular oil changes and everything it used to have kind of a rough low idle, but for some reason, it has randomly gone away. I’m not sure why I have done a lot of things to try to fix this gas mileage. I’ve changed my fuel injectors I’ve used fuel additives. I’ve cleaned the MAF sensor. I’ve cleaned the throttle body. I’ve changed the PCV valve. I’ve checked for vacuum leaks. All of my tires are at the correct psi. I’ve checked the air filter. I’ve changed the spark plugs. I have done all this and have not seen an increase in miles per gallon. I’m getting around 20 to 22 on the highway once again, my car drives perfect and shows no signs of any problems. The idling has gone away and now I’m just kind of stuck with this poor gas mileage. I brought it to a mechanic and paid $120 for them to tell me absolutely nothing and say they couldn’t find anything wrong. I have wasted my time, I’ve wasted money throwing parts at it money I don’t have trying to save more money on gas, I don’t know where to look next or what to do. My car is not misfiring. My car doesn’t have any smoke coming out as exhaust. My car is not shaking and appears to be in excellent condition as far as how it drives, but except for the gas mileage please if there’s someone who could help me figure this out because I got this car for liability number one and for its gas mileage number two so it’s been a real bummer not being able to get the full potential out of this car. I do all my work myself so if someone could please point me to the right direction or maybe it’s a common issue or experiencing the same thing, please let me know. Thank you.