2004 Chevrolet Trailblazer P2135, P2138, P300, P303 - Reduced Engine Power & Rough Idle Diagnosis

I'm experiencing a sudden and concerning issue with my 2004 Chevrolet Trailblazer. The truck has been running smoothly for about two years with no prior problems—excellent reliability overall. Approximately three weeks ago, while pulling out of the driveway, a 'Reduced Engine Power' warning light appeared on the dashboard, and the vehicle would only accelerate to around 10 mph. At that time, the OBD2 diagnostic scanner showed codes P2135 and P2138. I had a technician inspect it, who identified a damaged wiring harness and repaired it. After the repair, the original P2135 and P2138 codes disappeared and the warning light turned off—however, the engine began running very rough with a flashing Check Engine Light. Two new codes appeared: P300 and P303. I replaced the spark plugs, but the rough idle persisted, and the truck now only shows code P303. Prior to this issue, the vehicle ran so smoothly and quietly that it was nearly silent when idling—something I never noticed before. Since then, I've kept the 2004 Trailblazer parked and been using my other vehicle for daily needs due to work demands. While I can perform basic maintenance like fluid checks and wiper blade replacements, I'm dependent on a proper diagnosis from a qualified technician. I want to avoid repeatedly replacing parts without clear direction—any insights or suggestions would be greatly appreciated! Thank you, Jen.