> The president's job is to be the dumb unthinking agent of Congress

Execution will never be as simple as the textual content of a bill, so I'm not sure 'mindless' is a useful term.

While the POTUS is expected to be a bureaucrat, functionally and conceptually that is only one aspect of their responsibilities. Military oversight and diplomatic coordination as head of state, comes to mind.

I'm oversimplifying, but when it comes to the budget I'm pretty accurate: the president can't just decide they won't spend the money because they don't like where it's going or who it's going to. They certainly can't extort the lawful recipients of the money.