OBD2 Code P0420 on 2004 Chevrolet Optra: How to Fix Exhaust Catalyst Converter Issue

I have a 2004 Chevrolet Optra. One day, the car started running poorly with loss of power and intermittent stalling. The engine light came on shortly after. Using an OBD2 scanner, I retrieved code P0420, which indicates a faulty catalytic converter. After inspecting the exhaust system, I found that the front converter was completely blown out, allowing exhaust gases to pass through into the rear converter. I replaced the rear converter with an aftermarket unit, but the front converter is not available from dealerships—when I tried to order it, they quoted a price of $900. After about a month, I purchased a used front converter and installed it. The check engine light turned off and stayed off for roughly one week before returning again with the same P0420 code. My wife has now noticed intermittent misfires. Since catalytic converters are dealer-only parts (currently priced at $300), I'm unsure whether to invest more money or try a different solution. Can anyone provide guidance on diagnosing and fixing this issue without overspending? The original problem is clearly related to the exhaust system, but I need reliable advice before replacing another part.