Hitler should have just stopped the war after May 1941 and just fight UK and take her colonies and use his pact with Stalin to co-capture the Middle-East and finish off India. After 10 years of consolidation of capturing most of Europe and finishing off UK, invade the Soviet Union with the full enjoyment of all the jet fighters, super heavy tanks and greater war making capacity.
So i guess invading Soviet Union was the worst mistake and not using them as true allies until you didn’t need them anymore and causing a two front war.
Hitler should break any military treaty with Japan and try to 5th column USA with sympathizers to keep US neutrality.