As for a sustainable OOB, you need enough units on the front to provide a continuous line of divisions, plus be able to mass at least two sizable groups for breakthroughs. Those groups then push forward, drive at least 3 provinces deep, and then turn in to create a pocket before the units on the front line can escape through the last open province. The moment you close the pocket, expect severe all-out attacks from both inside and outside the pocket, with spoiler attacks on adjacent units that might be able to relieve those holding the line. I much prefer to create deep pockets, with a 2-province thick wall of units to reduce multi-direction attacks and massing against any individual province. Air units then attack the enemy units attempting to break the encirclement, causing them to wear down quickly, until you can reduce and eliminate the pocket. That can net you 10-20 units at a shot, and then you can let your units recover while you set up for the next encirclement. As the Soviet Army gets reduced, you may not need as many units to continue the drive, or can limit your advances to specific targets to push them over the edge. Pushing the Soviets back across a broad front (which the AI is programmed to do), rather than overrunning or encircling and eliminating, will almost inevitably result in Germany running out of Manpower, and losing.
At this point, I much prefer running minor countries, to see how much effect I can have on the bigger picture despite a severely limited set of tools to work with. The system is easily exploited, such as placing a wargoal for a region that your bigger ally is occupying, thereby getting freebie land that you didn't take yourself, so I have my own house rules to limit what I can and can't do, or conditions that need to be met before I can do something.