But Germany was going to be a cake-walk? Your reasoning seems to be based very much after-the-fact.
America did not expect Okinawa to be so difficult. You can read the contemporary opinions. It’s right there. Okinawa was a shock, and changed how the US saw the war. Again - it’s not difficult to understand.
Okinawa was difficult, US realizes that an invasion of Japan would be even more difficult. US steps up atomic bomb research.. Trinity comes a month later. Bombs developed for us on Japan and used.