P2178 and P0420 Diagnostic Issue on 2006 Cadillac STS 3.4L - O2 Sensor, Fuel Trim, Catalytic Converter

My 2006 Cadillac STS 3.4L is showing both P2178 and P0420 diagnostic codes. Initially, the vehicle displayed a P0420 code indicating catalytic converter inefficiency on Bank 1. To systematically troubleshoot, I replaced all spark plugs and tested each coil individually. All fuel injectors were also replaced with new units. Both upstream and downstream oxygen sensors (O2 sensors) before and after the catalytic converter were replaced. The catalytic converter on Bank 1 was subsequently replaced with a new unit. Additionally, the MAF sensor was replaced with an aftermarket part; however, I cleaned the original sensor and then reinstalled it. Despite these comprehensive repairs, neither the P0420 nor the P2178 code has been resolved. The P0420 code no longer appears because the P2178 code now surfaces first. Current readings show that short-term fuel trims on both banks are near normal (slightly rich). Long-term fuel trim on Bank 2 is around -6, while Bank 1 fluctuates between -14 and -28. When the long-term fuel trim drops to -22, a P2178 code is triggered. There are no detected misfires. The vehicle runs smoothly at highway speeds with quick and responsive acceleration. However, it exhibits rough idle and has difficulty starting, especially in cold weather. I am now seeking advice on the next troubleshooting steps to resolve this persistent issue involving P2178 and P0420 codes on my 2006 Cadillac STS 3.4L.