Even pre-war, USSR's production was still higher than Germany's one.
Your statement about USSR not being the economical powerhouse until after WW2 is not very true.
USSR would have been able to defeat Germany on its own. It just would take a year or two more and a few more millions of Russian lives to do it.
Germany had allies. At the end of 1941 the ratio of Axis to Soviet available population and economy was 2-1. And that was a German economy that was blocked from all sea trade by the oft-overlooked British blockade. I wouldn't just say that it was possible that the Axis would have been able to beat the Soviets alone, I'd say that it was all but overwhelmingly likely. The Soviets were taking casualties at a worse then 1-1 rate and the Axis had more troops to sacrifice.
If the war had drawn onto 1946 the Soviets would have had to go from inducting 6 million men a year into the armed forces into inducting 1 million men. They simply didn't have more manpower to draw. As it was they suffered a famine in 1946. If they had been fighting the Axis alone and the war was still going the famine would have been worse. Germany ran out of manpower historically but they would have run out if Italy hadn't surrendered (to say nothing of other allies) and they didn't need to bother with North Africa+Normandy+3 million men on anti-aircraft and repair duties.
It would have been a long, grinding affair. The Germans were in a horrible logistical situation that meant they couldn't achieve blitzkrieg successes. It would have been a war of attrition. But the Soviets despite the myth of Soviet hordes did not have infinite manpower and that manpower would have been exhausted first.