If Russia moves everything to the coast on turn 1 (or usually turn 2) they are going to get attacked. I’ve seen this go badly, and waste japans momentum (though its not costly in terms of TUV really), but there is a higher chance that Japan just annhiliates you because they can bring 5 air pairs and 2 sbs minimum plus a ton of land guys. Japan cant empty Manchuria until Russia wanders away or gets destroyed, but you may be handing him that opportunity, and killing those 20 Siberian Russian units is a huge benefit to the Axis (since they can save Moscow or stymy japan, in theory).
If Russia moves less than everything near Japan, its EVEN MORE likely to draw a big attack. 12 men are even easier to kill than 20. 3-4 guys isn’t worth activating the Mongolian rule for. As Russia you may see this as a brawny stack but its not. It cant attack and it can barely defend–certainly not against a power that has 20 planes!
The most direct way of winning is Operation Crussia. This plan is to have Germany and Italy leapfrog and kill Moscow by turn 6. Japan helps by taking the 7$ worth of eastern territories at the expense of probably not killing India or getting Spice Islands as fast. Japan can help stratbomb Moscow too. When all 3 axis work together, Russia is a lost cause.
As Russia, you want to make Japan think twice about entering your territory at all. You still need those men in the Moscow defense. As a result, they usually walk home, blocking the blitz in their wake. Japan won’t usually see it as convenient to attack you unless by doing so they can kill a substantial portion of those 20 guys. If they can do that, they grabbed your income and your rescue force in one blow. If the 20 guys head straight for Moscow, Japan may want to grab Siberian East $$ anyways, but once he starts, he should be facing 20 or so MORE Mongolians and Chinese partisans and this is where you want Japan to be stretched all over the place, having to fight 4 separate powers in 3 directions, only to face a confrontation the strongest and most powerful enemy (USA) at the moment he can’t take on any more. So I usually leave 3-5 guys in the east just to force japan to fight for every territory and use combined arms and planes in every battle–eventually something will give (or the Allies are going to lose since all of them, including the USA, are individually weaker than Japan until turn 8)
So, if Russia is standing where you can hit him, delay your plan and do a J3-4, focusing on taking out China and Russia early and building factories and land units.
If Russia isn’t being that bold, then the border should probably just stay as it is because Japan has so many enemies to deal with. Russia wants this state of affairs because it can’t spare the mobile units needed to help in the east and it cant take its eastern territories back once Japan rolls in. A J1 plan simply wont permit a Russia attack at the same time. So, if I see Russia stack up on the coast, I defer my J1 and kill them first.