1997 Hyundai Accent P0422 Code: Exhaust System Issue at 170,000 Miles

My 1997 Hyundai Accent with a 4-cylinder engine has always run smoothly and has consistently passed smog tests. It currently has around 170,000 miles on the odometer. For years, I could wipe my finger in the tailpipe and see no black residue — a clear sign of clean exhaust emissions. Recently, however, I've noticed significant black soot buildup inside the tailpipe. While the engine still runs well, fuel efficiency has noticeably decreased. Additionally, there's now a strong exhaust odor entering the cabin, which becomes more pronounced during deceleration. The first sign of trouble was this unpleasant smell. Shortly after, the Check Engine light activated, and the OBD2 scanner returned only one code: P042-2 (Catalyst System Efficiency Below Threshold Bank 1). About six months ago, I experienced misfiring issues, which were resolved by replacing spark plugs and ignition wires. This new issue appears to be separate. My initial suspicion is that the catalytic converter may be failing — a common cause of poor fuel economy and exhaust odor due to restricted exhaust flow. However, I'm wondering if it could also be an oxygen (O2) sensor malfunction? If the catalytic converter were clogged, that would explain the smell, but I don't observe any engine hesitation or sluggishness. I’m concerned about spending a large amount on a new catalytic converter for a vehicle over 20 years old — especially if it doesn’t resolve the issue. Any insights or suggestions from experienced owners or mechanics would be greatly appreciated. Thanks for your help!