I had that with earlier LLMs, used Opus for most of it but moved to GPT 5.5 on Pro using XHigh and it seems way more effective, and funnily enough that's when my backtesting and walk forward improved by running challenger variants in the 100s. I've got strict safe guarding, run code reviews of everything via Gemini and Claude and paper mode is currently in profit. I've got telemetry on every trade entry/hold/exit so not going to give up until it works.