Invading Russia wasn’t Hitler’s major mistake. Germany could have prevailed, or settled for a draw if it was fighting Russia alone, just like it did in 1917. Germany didn’t shift to a total war economy until 1942, and their military production increased throughout 1943.
No the major mistake was declaring war on the United States. There was noting to be gained — Japan was in no position to assist in his aims. He was blind to the industrial potential of the US, he didn’t anticipate thousand bomber raids on cities like Hamburg.
Another big mistake was allowing all the atrocities in captured Soviet territory. If the Germans had taken a milder hand, it would have encouraged more Soviet people into fighting on the German side.