It sounds like you're looking for the field of psychology. And like the field of psychology, any predictive abstraction around systems this complicated will be tenuous, statistical, and full of bad science.

You may never get a scientific answer to "how should I prompt the model to achieve <x>", just like you may never get a capital-S scientific answer to "how should I convince people to do X". What would it even mean to "understand people" like this?

You demand too much.