To be honest, I don't think the german stratgy will work. The Soviets reach your MLR, build up forces, bring in supply, overhelm your defense, you retreat, play this again. If you only have 2 lines left, it will cost him lets say 20 turns. Maybe 30. Not enough. Either you defend more aggressivly or you plan more defensive lines.
I'd massivly shorten the lines. His numbers don't profit that much - he may overfill his hexes, increasing his losses. But he breaks through anyway, so why bother with that? But you would have more reserves and would be able to launch more counterattacks or build more lines.
At least in my theory