I don’t do this move as the Soviets because Japan starts with too many units in Korea/Manchuria and too much aircraft in range of Amur.
I think it’s a mistake for Japan to use its fleet/transports to attack Amur J1–they can still attack without any transports and divert a strategic bomber to Yunnan and still have 100% odds with an average +25 tuv swing. If they send their fleet and transports south they’ll be in good position for a J2 or J3 declaration, but if Japans’ transports/fleet are stuck in sz6, Japan will have to put off its declaration till J3.
But I’m uncertain about it because however good this attack looks for Japan on paper, it will divert alot of troops from China. The extra 6 inf in Mongolia help make up for the Allied tuv loss. But the Soviets will also lose out financially because Japan will conquer Siberia quickly.
Massing Amur on R2 seems like the better move because the bulk of Japan’s airforce is usually out of range of Amur on J2.