I think the bigger issue is when he did and not that he did it. He was already getting reports that Operation Typhoon was being stalled and that they were no closer to winning at Leningrad so it makes zero sense to get the United States directly involved when you are starting to see a far more competent and fiercer Russian resistance than you saw in October starting to emerge outside of Moscow and Leningrad. I think that Hitler was always going to be at war with the Soviets whether he started it or Stalin started so 1941 was probably the best time he could have started it. But there was really no real assurance that the Americans would be directly involved by 1943.That is an interesting question. The Japanese attacked Pearl Harbor 7 DEC 1941. Germany declared war on the US on 11 DEC 1941. What if he had not.
I think the US would have continued Lend-Lease to the Brits and Soviets (probably by sailing through the South Atlantic to the Indian Ocean to Suez (for the Brits) and Iran (for the Soviets). Hitler might have tried to interdict that using U-boats and that would have, and then the US would be involved in Europe anyway.
Or, alternatively, the US could have sailed right through the North Atlantic and dared the Germans to attack them, which they probably would have done.
I think with Nazism leading the war that Germany was always doomed to lose, bringing in the United States just fast tracked the date by a couple of years.