The spring rain made an earlier start impossible, especially since the German plan depended on an early rapid advance to encircle SU armies before they could withdraw. With more men and resources the German would capture more land but if lend-lease is comming to SU Germany will lose the war.
If Britain hadn’t been fighting a war in Europe Japan would have lost badly, they didn’t even capture India when it was Britains 3. priority. The japanese knew this and would not have stated the war in those circumstances.