How do you suppose that he got away with
instant tyranny, as much as he wanted? (For pedantic correctness: Off the top of my head, I don’t know if the Roosevelt gold ban was one of the issues
directly impacted by the court-stacking threat; but I do know that the Commerce Clause abuse that is nowadays institutionalized to Federalize everything did start with Roosevelt, for exactly that reason. And your Supreme Court should have, and otherwise would have promptly torn up everything from Socialist Insecurity, to quotas imposed on how much farmers could grow, etc., etc.)
A president can't just pack the courts though. His only real power is to stop them from being packed. For the number of judges to change, you need enough of the 400+ members of the House that represent the entire country and are elected every 2 years to make it happen. So, not really tyranny.