I've been contracting with a startup. The bottleneck is not the lack of tools; it's agency. There's so much work, it becomes work to assign and organize work.

But now who's going to do that work? Still engineers.

It sounds nice, but are product managers able to spot regressions or other potential issues (performance, data protection, legal, etc) in the codex result?

If codex can analyze the whole code base, I can’t see why not? I can even imagine one can set up a CI task that any committed code must pass all sort of legal/data protection requirements too

Exactly this. In fact the product manager should be the one that knows what the set of checks that need to be done over the code base. You need a dev though to do make sure the last mile is doing what you expect it to do.