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.
Found it - https://youtube.com/watch?v=yOWX9LVUt2w
It says that the US did most lf the fighting, so felt entitled to take the lion's share and were the only ones with the A-bomb so felt they could dictate what they wanted. And the Allies were not in a good financial position to sustain another occupation. It makes some degree of sense, but I don't understand why the exact same arguments didn't apply to Korea just across the sea.