Stacking in Amur is suicidal. Japan can reach it too easily and with too much. You’ll get a half dozen hits or so before being flattened by the Japanese airforce and once those troops are gone, the far east will be blitz’d quickly and the Soviets will be starved far worse than usual. Unfortunately, having just infantry and AA means you’ve got very little offensive power and Japan’s enormous airforce means you’ve got even less defensive ability. So their job is really to make the best out of a bad situation.
I send the 6 INF closest to Moscow back. They’ll be essential and make it in time if Germany decides to go head on for Moscow.
The other 12 INF 2 AA will be moved to Buryatia. Any further forward and a Japanese attack on Amur will leave them cut off and as good as dead.
On R2, I’ll send 2 AA and, depending on Axis play, possibly 6 INF back to Moscow. If the Japanese airforce hits your infantry the stack is dead, so the AA are next to useless there. They might make a huge difference in a German attack on Moscow though.
For the rest of the game, those 6/12 Infantry’s job is to force the Japanese to commit. They’ll need to move significant air/land forces in position to hit the stack, at which point you retreat. Then repeat. Every step it makes forward forces Japan to commit enough of it’s troops and have planes in range to attack a 6/12 Infantry stack, and enough troops to defend against an attack from one since it can’t land it’s aircraft after moving forward. This helps take pressure off India, China and The Pacific.
At Yenisey, Japanese troops will be in range for your fighters/tac(s) out of Moscow. Probably, Japan will have 4-6 ground units - possibly with some following further back. In other words: they won’t have enough to push any further forward without being destroyed in a counter attack. If you get lucky, they’ll move forward and die. If not, you’ll trade Yenisey back and forth with small attacks for a couple turns. Either way, buying time and killing scarce Japanese ground forces is really the best you can ask for.