A few of my balancing ideas:
The number of UK units mobilized in India can never be more than half the number placed in UK.
If Russia is knocked out, the CPs get a number of free units to represent freed prisoners of war. This could be extended to any power being eliminated, but applies more usually here.
USA does not collect money or buy units until it is at war. One first turn at war it gets a “draught bonus” and can but infantry at 2 for that turn only.
To represent the central European rail system, the CPs can transport units up to 3 areas within their original European territories.
As above, Munich is a production centre, but losing Berlin, Munich or Vienna results in an instant CP defeat.
CPs can create red “Bolshevik” infantry units in occupied Russia. These will fight “White” Russians and any other Allied units in original Russian tt.