I think it depends if the Russian Revolution Rule is in play. That will alter the strategy somewhat. Heres the deal: Germany is forced to fight on both fronts. Russia is too strong for AH to take on by themselves. You need infantry. Forget about planes, artillery, or naval. Germany starts the game with plenty of artillery, so it is unnecessary to buy more. You need infantry to replenish your losses. Also you need infantry to contest and occupy Russian territory to bleed the Russians dry.
The Russians need to choose who they will deal with. It will be either AH or Germany. Since AH is closer to Moscow, it is wiser for them to fight an offensive war against AH and a defensive war against Germany. As AH is slugging it out with Russia. Germany slowly squeezes the life out of Russia by draining her economy. It will take Germany many turns to accomplish this, but once finished it gives them the needed income to hold the line on the Western Front. By this time, Germany has probably lost Belgium, and the Allies are contesting the Ruhr. Remember, you have to give up something to get something.
The Germans should not send the forces deep in Russia back to the Western Front. It would take too long. Those forces should go to put pressure on the UK in the Middle East.