I know house rules may not be the answer, but I have one that might. I follows the World At War game rule. The later versions where Russian is not considered a “friend or foe”. That rule does not allow any ally troops into Russian territories as was the case in the real war.
In fact Russia would not even allow fly overs or allow bombers to land in thier territories so the allies could conduct bombing runs deep into their territiores.
And for those who think they are history buffs who may contrdict me with some obscure example, let me give you one I already now. I don’t remember the exact places or years, but thier was one instance where they did let bombers land in Russian territory. The Russians kept them for safe keeping then gave them back stripped. They were looking for technology they could use.
It gives some realism. They also don’t liberate anything. If the Russians take it from the axis, they keep it. This should help in terms of not allowing the Russians to be reinforced. It also would make it much less advanages to buidling an IC.