Have you tried Deep Think? You only get access with the Ultra tier or better... but wow. It's MUCH smarter than GPT 5.2 even on xhigh. It's math skills are a bit scary actually. Although it does tend to think for 20-40 minutes.

I tried Gemini 2.5 Deep Think, was not very impressed ... too much hallucinations. In comparison GPT 5.2 extended time hallucinates at like <25% of the time and if you ask another copy to proofread it goes even lower.