Japan's political environment during the war was incredibly complex, and trying to view their rationale and motivation through a Western lens is very dangerous. And I don't think the Soviets were anywhere close to an imminent invasion of the Home Islands. I don't think they had the appetite for doing more than grabbling the Kuriles and Sakhalin Island, instead preferring to let the US suffer the casualties.
While the Soviet declaration of war surely had some impact, the Emperor said there were three primary factors in his decision to accept the Potsdam conditions; his lack of confidence in Ketsu Go plans to defend Kyushu. The increasing devastation caused by the conventional and nuclear bombing campaigns, and finally, concern about the "domestic situation" meaning internal revolt. Later in private letters he referred to Nippon's deficiency in science, meaning a lack of nuclear weaponry. Hirohito and PM Suzuki realized that with nuclear weapons, the US didn't need to invade Japan.
Strategic bombing has only influenced one country to surrender.