03 Ram 1500 hemi “system too rich”

So I have an inherited truck: 80k km that started throwing system too rich.

Started with plugs, then did injectors (mopar NOS), gutted the blocked cat, new O2 sensors. Cleaned the MAF, new air filter.

System to rich left, rough idle and random misfire showed up. Sent it to my wizard. Cleaned the valve covers and checked valve clearances, checked all wiring and sensors, couldn’t stop it. Got the misfire solved but timing started going all over the map at idle.

Swapped ECU. Back to “system too rich”. 🤦‍♂️

The cat-gut guy mentioned the upstream o2 sensor might need a bung. Will this help? I can’t see how pulling the sensor out of the exhaust flow changes the particulate detected (granted I’m that level of ignorant on the subject.)

Any next steps or ideas? Other than running the risk of carboning-up all the innards, is there another reason not to run rich?

Would using some kind of aftermarket tube solve anything? Clearly I’m out of ideas and ready to grasp all the straws.