Meanwhile codex is running a free tier for a month right now for anyone who wants to experiment with it: https://openai.com/index/introducing-the-codex-app/

Doesn't appear to include the new model though, only the state-of-yesterdays-art (literally yesterdays).

I have access to 5.3 in the Codex desktop app. I'm really not pleased with the desktop software (insane performance issues) but the LLM is good.