I see that you got some responses from people who may have not even used gpt-4 as a coding assistant, but I absolutely agree with you. A larger context window, a framework like Aider, and slightly-better tooling so the AI can do renames and other high-level actions without having to provide the entire changeset as patches, and tests. Lots of tests. Then you can just run the migration 15 times, pick from the one which passes all the tests... run another integration pass to merge ideas from the other runs, rinse and repeat. Of course the outer loops will themselves be automated.

The trick to this is continuous iteration and feedback. It's remarkable how far I've gotten with GPT using these simple primitives and I know I'm not the only one.