This was my experience too until a couple weeks ago, when Codex suddenly got dumbed down.
Initially, I had great success with codex medium- I could refactor with confidence, code generally ran on the first or second try, etc.
Then when that suddenly dumbed down to Claude Sonnet 3.5 quality I moved to GPT5 High to get back what had been lost. That was okay for a few days. Now GPT5 High has dropped to Claude Sonnet 3.5 quality.
Initially, I had great success with codex medium- I could refactor with confidence, code generally ran on the first or second try, etc.
Then when that suddenly dumbed down to Claude Sonnet 3.5 quality I moved to GPT5 High to get back what had been lost. That was okay for a few days. Now GPT5 High has dropped to Claude Sonnet 3.5 quality.
There's nothing left to fallback to.