When I do West Russia/Belorussia, I leave Caucasus pretty much open. Like 3-4 infantry. No AA gun.
If Germany wants to stratbomb, great, that’s something else that didn’t get hit (like Anglo-Egypt).
If Germany wants to invade heavily, great, they can attack with everything they have, and I can still kick them out with massed W. Russia/Russia forces next turn.
If Germany wants to invade lightly - well, it’s not easy even then. If Germany wants to attack without committing tanks, it has to use air, and you know how tricky it is to allocate German air on G1. (Even if the Germans send the Med fleet west, if German fighters hit Caucasus, they MUST land in the east, which lets the Allies get a turbo boost on setting up their transport fleets, without the fighters in W. Europe to worry about that first turn). If Germany DOES commit artillery and/or tanks, well, I just traded Russian infantry for German infantry, artillery, and/or tanks. And what’s more - I traded Russian infantry for FORWARD MOBILIZED German infantry, artillery, and/or tanks. Quite nice.
Russian fighters land at Moscow. This deters a German transport build in the Baltic on G1.