DISCLAIMER: This is where much of the "Handwavium" had to be spent, the concept of a German victory on the Eastern Front is sketchy at best and Stalin's incompetence really had to be increased to 1945 Hitler levels in order to make this work. Assume the absolute worst case scenario had to happen with the USSR here, such as the factory relocation to the east starting off too late, a bigger siege of Kiev, a German focus in capturing the Caucasus instead of Typhoon, etc.
In the end, for story-purposes, great liberties had to be taken in order to make the Soviet defeat to happen, keep it in mind before going into the commentaries and accusing me of being some sort of Wehrmacht supporter.