I've had this issues as well since codex models were introduced. i tried them but 5.1 regular on high thinking always worked better for me. I think its because its thinking is deeper and more nuanced it seemed to understand better what needed doing. I did have to interact more often with it versus Codex which just worked for a long time by itself, but those interactions were worth it in reduction of assumptions and other stuff Codex made. Im gonna try 5,2 Codex today and hope that changes, but so far I've been happy with base 5.1 high thinking.