What does the paid diagnostic add?

Two things. Scope: it covers the site rather than the page, and most visibility problems are not on the page you would think to test. And remedies: the free reading names the heaviest three problems and stops, while the diagnostic names every failure, what to do about each, and the order worth doing them in.

Between 19 and 49 dollars depending on page count, quoted before anything is charged. If the honest answer is that the free reading already told you the thing that matters, you get told that instead. What it covers.