That's a pretty good point that I haven't considered before.

In about a quarter of the timelines, the US would have joined them.

The Manhattan Project, which lead to the creation of nuclear weaponry, was only started in 1942, which was after the Pearl Harbor bombing in 1941. It was started as a response to entering World War II, not as a precaution. Had Pearl Harbor not been bombed, it is doubtful the Manhattan Project would ever have been started. The US seemed to not want to get involved until Japan forced their hand, so it is likely the US would have remained outside of the conflict had Japan not attacked.

Had the US remained untouched by World War II, and assuming Germany somehow gained control of every country in Europe, Asia, and Africa, it is doubtful the US would have had enough time to complete the Manhattan Project. And even if they did, what would it gain them by nuking Germany? At that point Germany would already control half the globe. Surely they wouldn't nuke German occupied countries and catch millions of innocents in the crossfire? The only thing they would realistically be able to do is try to keep Germany on the Eastern Continents and heavily fortify Alaska.

Look, all things considered, the actual history is probably pretty good compared to the nightmareland that could have been if Germany wasn't so self-absorbed.