I play Japan quite a bit and what I’ve realized is that Japan basically has four areas that it can put resources into, but it only has enough money to focus on 3 of those areas. The areas are the Pacific, the DEI, China/Southeast Asia, and Russia. Now it’s not impossible to do all 4 and sometimes the Allies let you get away with it, but I usually plan to focus on 3 of those
Now I usually always put resources into the Pacific (US fleet) and the DEI. Which leaves you a choice of going after Russia or Southeast Asia. I usually focus on China and southeast Asia since if you let that area get away from you they’re going to sweep up and take away all of your coastal territories (which you can let them do to some extent if your goal is to sweep across Northern Russia as fast as possible). But if you take out the Chinese now you can threaten Russia on 2 fronts while removing one of your enemies.
For the question of when to attack Russia it really depends on what they’re doing. If they stack all of their forces in Amur on Round 1 send everything you have to take them out. They’ll only try that once. Also, if they stack a huge force in Sakha on the first round, then unload everything you can into Amur on your first turn and put a bunch of airforce in range. Now you have them trapped since they can only move to Buryatia, you can lose a fair amount of airforce in the attack but the whole of northern Russia is now yours.
A round 2 attack on Russia can be pretty effective since you can take all of the coastal territories and push them back pretty effectively.
One last thing, if your airforce is in Kwangsi and China has taken back Yunnan with minimal force and stacked in Szechwan, then send your whole airforce to attack Szechwan. You’ll wipe out all of the inf, and China will lose it’s only fighter. I just did that in a game and took out 5 inf, 1 ftr for the cost of 1 of my fighters. Definitely worth it.