1998 Honda CR-V P0420 Code Recurring After Multiple Cat Replacements and Flex Pipe Installations

I purchased a 1998 Honda CR-V three years ago (with 118,000 miles at the time) from a dealer in Houston. About a week after purchase, the Check Engine Light (CEL) activated with fault code P042-0, indicating a catalytic converter efficiency issue. The letters 'CEL' were previously written on the windshield corner, but I initially dismissed it. I returned to the dealership where their technician diagnosed the problem as a faulty Bank 1 O2 sensor and replaced both sensors. After driving approximately 700 miles, the P0420 code reappeared. The dealer then claimed it was due to a failing catalytic converter (cat), and they installed a welded-in universal cat—though this significantly altered the original OEM system by removing factory flanges. The vehicle ran well for about three months before the P0420 returned again. I took it back and requested that the same technician re-evaluate, suggesting he install the catalytic converter closer to the exhaust header (approximately 12 inches downstream from OEM position). This fix worked reliably for nearly two years until at 154,000 miles, the cat cracked due to the rigid welded installation—no flex pipe was used. Recently, I had a local mechanic in Austin install a new catalytic converter. He used a Cherry Bomb XL (designed for high-mileage or problematic vehicles) and added a 4-inch flex pipe. Additionally, he installed an O2 sensor bung after the cat instead of using one with a built-in sensor, to provide more space for optimal catalytic conversion. However, less than 300 miles later, the P0420 code returned—no misfire codes or other drivability issues are present. The vehicle runs smoothly, doesn't burn oil, maintains excellent fuel economy (better than factory ratings), and shows no signs of rich or lean mixtures. My question: Can a catalytic converter be failing without any associated misfire or performance symptoms? Given that the car otherwise performs well, is it possible the original cat was already degraded before replacement? Additionally, my inspection expired this month, and I haven’t been able to get a valid sticker due to repeated CEL issues. What are reliable troubleshooting steps for persistent P0420 on a 1998 CR-V? Any recommendations for proper diagnostics or component replacements?