Change the game into “Axis, Allies, and Comintern”
Russia gets it’s own victory conditions, and permission to negotiate with the allies or the axis as required for peace/war/etc.
Suddenly the setup needs no balance at all, as 3 powers vying to dictate their will onto the board - creates a balance of it’s own.
Invariably, the same results of WWII occur, BUT - now you have some balancing factors!
I would set the bar for “Russian” victory at how many IPC’s they earn, and/or control of certain territories - like Mongolia, Berlin, Finland, Poland, perhaps turkey or the middle east as options.