Horrible fuel economy, backfires/crackles/pops near the CAT at above 4,000 RPMs?

I have the car listed above GTS. I changed the spark plugs, and the tips were light brown indicating they were fine. I have very bad fuel economy around 18MPG when my car is supposed to average 30-33MPG as my friends have the same car. The car backfires/crackles/pops near the CAT at above 4,000 RPMs which is causing damage to the CAT and shot out a pending p0420. The backfiring sound never seems to happen below the 4,000 RPMs range. O2 sensor seems to work fine as LTFT and STFT change with throttle response and changes voltage. Once in a while occasionally I’ll get a pop from the intake. When accelerating at low rpm’s, the car bogs/jerks when pressing throttle and bogs/jerks when releasing throttle which can also be seen on the RPM gauge slightly. Sometimes when I take the car out of gear and put it into neutral the rpm will have a surge dancing back and forth quickly from 1,100-500 RPMs and goes back to normal usually when I blip the throttle but doesn’t stall. My LTFT is sitting at -8 to -10% and sometimes STFT might jump to -15% for a second or two. When accelerating sometimes it’s either positive LTFT of 6ish and sometimes negative 6 to 8ish. What could be my problem? I have cleaned the MAF and TB so that is good, and the ignition coils should be good too as no misfires and no engine cel codes other than p0420 which is due to the backfiring damaging the CAT slowly. Also did a smoke test and didn’t seem to find any vacuum leaks or evap leaks. I really need to get this diagnosed whether it be a leaking fuel injector, loss of compression, intake clearance out of adjustment, timing chain maybe jumping a tooth, exhaust leak that’s not visible/audible, whatever it may be, I want to get to the bottom of the cause and have my fuel economy back.