“every single direction change I suggest, codex is all: "that's a great idea, and we should totally go that [very different] direction", try as I might to get it to act like more of a peer.“

That’s not the model, that’s a personality setting you can change in the codex config file.

Set it to Pragmatic, and ask it (not command it) about your new direction in planning mode.

It will tell you if your idea is not good for the given project. It’s an excellent peer.