In your case I’d strongly suspect that the catalytic converter is reaching the end of its useful life.
The converter core is a ceramic honeycomb coated with a platinum-palladium (an alloy in the rhodium family). When the coating is heated by the exhaust, it strips the NOx (which is nitrogen atoms bonded to oxygen atoms) or its oxygen. The output of that first stage of the converter then becomes nitrogen (of which air is 77% anyway) and oxygen, which can then be collected by carbon monoxide (making carbon dioxide) and/or released as oxygen.
The problem is that only the NOx molecules that come directly in contact with the heated metallic coating undergo this transformation, and no matter how clean an engine operates, there will always be some carbon (soot) in the exhaust. The carbon over time coats the platinum-palladium, reducing its effectiveness, since the coating prevents the NOx molecules from coming directly in contact with the metal. Thus, the efficiency drops over time.
By all means check the EGR system out. That allows a wee bit of inert exhaust to be drawn into the intake under load, thus keeping the cylinder temps from becoming hot enough to ramp up production of the NOx in the first place. If cylinder temps spike, NOx production increases.
But also start saving for a new cat converter next year. Or in two or three years, depending on how long it takes the converter to get below the efficiency required to keep the NOx levels low. You might never need the money, but at least you’ll have had time to prepare in case you do.