More realistic would be an attempt to grab the rest of Sakhalin and see if they can get away with it while the Soviets are preoccupied. That, they might be able to do without getting torn to shreds by superior Soviet equipment and manpower.
Although I'm honestly rather surprised they didn't do that during the RCW.
Don't worry, they did. The West just forced Japan to give it all back.
And the Soviets MIGHT tolerate the loss of Sakhalin with hasty negotiations if the Germans are driving on Moscow. They would probably be willing to throw out more if Germany has grabbed Moscow due to Japan tying up all kinds of forces in the East.
Either case would be a boon to Japan, which can probably get Sakhalin oil to support its war machine, at least partially and possibly last long enough to wait out the allied Embargo.
That's huge, Sakhalin was exporting millions of tons of oil even back then.
It raises questions of there not being a Pearl Harbor.
Certainly, it eases Japan's pressure on making a hasty move towards war, even if all this does is gives it another year to make arrangments.
WW2 as we know it gets butterflied, though its hard to say how. Suffice it to say bombing the Sakhalin Oil Fields is going to be critical to the Allied War Effort if Japan decides to attack Pearl Harbor (which they probably will not do on schedule, and might pass on entirely)
Here's an idea: the Japanese move the army to the border not so much for an actual invasion as to provide a stick to use in negotiating the Non-Aggression Treaty with the Soviets. In TTL, let's say that Sorge gets caught before he can tell the Russians that they have nothing to fear from Japan, and the Japanese squeeze North Sakhalin out of Stalin in exchange for the Non-Aggression Treaty.