The Japanese were fighting with their arms tied behind their back while the Soviets were given the green card to do as they saw fit, as the Japanese high command did not want the border conflict to escalate to war. Even when the Japanese were handicapped they still inflicted on average twice their loses on the Soviets.
I'm not disagreeing with you. The Japanese seemed to generally outperform their counterparts man to man. The Soviets, however, had amassed such a pile of men and armor, and in a fairly short time, that the short and medium term fighting was essentially hopeless for Japan. They could of course shift more forces to the area, but a longer term, full born war with the USSR -and their 300 some standing divisions- was a losing proposition. The Japanese tactical performance, though laudable in some ways* didn't stack up to the terrible operational and strategic situation.
The Soviet attack, utilizing their rail and road assets, came as a complete shock. The Japanese didn't really view it as possible for the Soviets to build up so fast and so quietly. The Japanese were unable to match the build up speed, meaning that the massive force and firepower ratios in favor of the Soviets would continue barring the war moving onto Japanese territory. If the war did that, the Japanese were losing anyway.
*sorta... The 23rd division performed well when shooting at the enemy. However, they failed to communicate that they were under attack from 3 sides by a conglomeration that might be termed a combined arms army. They failed to attempt withdrawal until far too late. They failed to attempt any sort of breakout, at least regarding most of the sub units. To their credit, though, they stopped the Soviet cold in the frontal and Northern attacks. Some of this is probably due to the Soviets been somewhat inept, particularly in the Northern attack (If I remember correctly; it's been a while since I really dug into it, but I did have a class that focused on it rather heavily as a case in point for supply/operational warfare), but a lot of it has to be credited to the Japanese. They would go on to show similarly tough characteristics, also to little avail, in the Pacific war.
-------- (General thread reply, not specific to you, Mr. Lucifer)
Of course, that doesn't apply as much in the OP's situation. A 1941 attack would see the Soviets be unable to match the total force from Japan, particularly as Japan shuffled in more forces and the Soviets took losses. The IJN's complete naval domination of the area would grant some degree of mobility advantage to the Japanese for any coastal operations, though inland the Soviets still probably move around better.
The bigger issue there seems to be that it doesn't offer a fix for Japans current issues. Not oil, not steel, not good relations with the Americans, not reduced aid to Chinese opponents. What it does do is open up a massive meatgrinder that will pull in men and material faster then the fighting in China. All for what? Vladivostok? Occupying the rest of Sakhalin, a bunch of mostly useless islands, maybe grabbing bits of Kamchatka?
Worse, Japan is still going to war with the UK, US, French holdings, the Dutch and Australia. Suddenly there's a humongous shipping crisis, as all those beans and shells and boys going to Siberia need ships to get them to there from Japan.
Maybe, to play devils advocate, they could force a negotiated peace with Stalin in Asia. Hell, maybe even it shocks Ioseph so badly that his government fails or something.
I don't know if the success would actually help Japan, as they'd still need to keep the Kwantung army supplied and large. Meanwhile, their doom approaches from the Pacific.