A bad (or cheap) O2 sensor would contribute to a clogging converter. I’d replace the cat, put in a good sensor & clean or replace the PCV valve and get the oil leaks fixed before they get worse if you plan on keeping the car. Unless you’re planning on trading for a new car, it’s called maintenance. Take good care of your vehicle and it’ll be dependable. That stuff is all related. When you get up there in mileage an oil treatment additive every oil change and a decent fuel system cleaner every other tank, or 6 months does help maintain fuel economy, cleanliness, power and reliability. I’ll use a fuel treatment on a long trip. It doesn’t do as well on a grind of short commutes. It’ll also give you a better ownership experience for the vehicle. It sounds like you’ve got a decent mechanic, and you’ve kept up on your vehicle. Your Camry will probably love you for it.