In your opening moves on R1. Simply Declare a state of war with Japan. Without actually attacking or moving aggressively anywhere.
Russia instanty gets +5 IPC’s a turn no matter what happens, for the next 4 turns (Unless Germany attacks early). Thus, there is absolutely NO REASON not to declare war.
You can even tell Japan you are doing it just for the $5 a turn and that your not really going to attack them. and then position your units in the most effective defensive way possible.
Fly your planes into range if you like, for counter attacks on the following turn, because chances are the Japanese may try to take a territory just because.
This gives Russia something to do early, and dilutes the invasion of China every turn for the rest of the game, whilst Russia makes extra cash. Infact, because you are at war, you could roll a tank a turn into China, why not? It’s already paid for. China can even Can-open for you. If it gets bad, you get an armor boost near game end in your capital as you retreat. Better, put a Russian bomber in China somewhere, that will keep jap transports honest!
The opposite of this is pointless… even if Japan didn’t attack until R4, you’d see no extra money, and they can still attack you anyway, which will cost you the territories you lose.
Oh, and totally abandon Vyborg whilst you are at it. Since there is absolutely NO reason, to ever have a unit there, unless you are the germans trying to stop a blitz into Finland.