Engine misfire?

These symptoms routinely point to a clogged or malfunctioning damaged or failing catalytic converter, like loss of power /RPM drop and the smell of sulfer. Also repeated use of additives can damage the cat. No need for them theres plenty of cleansers in the fuel that insure the longevity of the engine save your money, that said not sure why your not getting any codes unless the o2 sensor and or cat have been bad for a long time maybe the wires are damaged or cut /disconnected. Usually youd get a code stating that theres fualy component80s so its odd that codes are not being cast. You said self diagnosed misfire ddid you scan it yourself? There should be a code for that and or anything else the might be acting up. Could be in limp mode or somthing but maybe have it scanned to check and see and let us know. Best of luck hope this helps.

I agree with MECHTRIX, I would use a rubber mallet and tap the exhaust system to see if you can hear any loose parts inside, this will tell you the cat has come apart. Also, if you use a scanner you can tell me the short and long trims it will help. It could have a weak fuel pump as well. Here is a guide to help test the fuel pressure, you can rent a fuel pressure test kit from the auto parts store for free in most cases. Please check the instructions in the images below.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Fuel Pressure .................... 30-45 psi (207-310 kPa)

Install the gauge in place of the fuel filter. Please go over this guide and get back to us.
Check out the images (below).