This is a bit of an Ask Historians question.
I’ve been reading about the Japanese surrender on Wikipedia, and one thing I thought was strange was that the post-war occupation of Japan was largely handled only by the US under MacArthur. The original plan during the war was apparently for the Allies to divide it, but somehow the plan changed. Stalin allegedly wanted to occupy Hokkaido, Japan’s northern island, but Truman was opposed and it didn’t happen.
Contrast this to Germany (an East-West split than lasted for decades) and Austria (an East-West split, but the Soviets didn’t block full Austrian independence after a relatively short period. In Asia, the Japanese-controlled areas were mostly returned - China received Taiwan, coastal China in the south and east and Manchuria in the northeast. The Soviet Union retook Sakhalin island, just north of Hokkaido. Korea had been occupied by Japan for a few decades, and rather than Japan, it was Korea that was split between the Soviets and the US and shortly after became DPRK and ROK, transitioning into the Korean War as we know it, and the Korean peninsula is still split.
Japan, I think, fared reasonably well - the US were largely gone within ten years (but with a presence of military bases), and even during the occupation, Japan still technically governed themselves. I think it could have potentially gone much worse if the Soviets were involved, but the reasons for Soviet non-involvement are not very clear.
By “Japan proper”, do you mean Honshu? I think yours is a good answer, but it still feels a bit weird. The Soviets invaded South Sakhalin in mid-August 1945 and finished on 25 August. From Sakhalin to Hokkaido looks about 30 miles. It wasn’t until 28 August that any US forces landed in Japan (excluding Okinawa). So I feel that the Soviets were in a good position to also land in Japan, assuming they had the numbers and transport means.
There’s also a section in the wiki article (https://en.wikipedia.org/wiki/Surrender_of_Japan) about how Japan were trying to get the Soviets more involved in the surrender negotiations in the hope that they could avoid unconditional surrender, but the Soviets were simply stalling so that they could maneouver into position in the Far East.
The division of Korea as also a bit weird in that they agreed to split across the 38th parallel, but the Soviets got to that line a few days ealier than the US, but upheld the agreement and didn’t go any further.
I’m not really concerned at all with when Americans landed in Japan. The US unilaterally established air superiority over Japan and had successfully blockaded the country, bleeding it dry of oil. The Japanese mainland was functionally militarily defeated prior to any land forces making it there.