I don’t know if I agree that the USSR was not in a serious position to invade Japan. For one, after they declared war on Japan as promised in the Yalta/Tehran Conferences, they were able to invade Manchuria, Sakhalin, and the Kuril Islands. I’m aware that the Kurils are tiny tiny islands compared to Hokkaido and Honshu, and they were locations Japan was probably not defending vehemently. So maybe the USSR wasn’t prepared for the scale of a naval mainland invasion, but given the Japanese losses in Manchuria, it certainly seems they at least had the manpower in the region. It’s not like the Red Army had to march all the way from Berlin to Vladivostok.

The Soviets and Mongolians ended Japanese control of Manchukuo, Mengjiang (Inner Mongolia), northern Korea, Karafuto (South Sakhalin), and the Chishima Islands (Kuril Islands).

This also seems to be a point of discussion among historians: https://en.wikipedia.org/wiki/Soviet%E2%80%93Japanese_War#Impact_on_the_Japanese_decision_to_surrender

American historian Richard B. Frank points out that there are a number of schools of thought with varying opinions of what caused the Japanese to surrender. He describes what he calls the “traditionalist” view, which asserts that the Japanese surrendered because the Americans dropped the atomic bombs.

Tsuyoshi Hasegawa’s research has led him to conclude that the atomic bombings were not the principal reason for Japan’s capitulation. He argues that Japan’s leaders were impacted more by the swift and devastating Soviet victories on the mainland in the week after Stalin’s 8 August declaration of war because the Japanese strategy to protect the home islands was designed to fend off an Allied invasion from the south and left virtually no spare troops to counter a Soviet threat from the north.

Btw, the story about how the surrender broadcast recording was saved from the coup, I always thought that was a fascinating story.

https://en.wikipedia.org/wiki/Hirohito_surrender_broadcast