Russia’s primary purpose is to keep Germany busy. I believe a lot of players interpret that to mean “hold Moscow”, but that’s only a means to an end and should be disregarded if and when abandoning Moscow is a more effective means of keeping the Germans occupied than holding it.
Personally, I believe Russia should keep it’s ART numbers at around 50% of it’s infantry. A stack of 20 INF 9 ART vs 32 INF, for example, has a very small loss of defensive capacity for an enormous increase in offensive ability. You’re not going to be able to take the German stack head on, but it will allow you to keep Germany honest: to slow them down, keep them from splitting their forces and taking multiple objectives, force them to use their air defensively, etc. Without the offensive capacity Germany will leave you bottled up in Moscow building 2 infantry a turn while it continues to seize more industrial capacity.
A defensive battle, with Germany’s entire airforce in play, is unlikely to be in Russia’s interest in any case.
In other words, the best way to keep Germany from the Middle East is to slow the German advance as much as possible in the first turns, and then to maintain your ability to push back West as soon as they try to swing South.