It’s hard to call Germany’s invasion or Russia a strategic mistake. They had no shot at winning, but that was the whole point of the war. Also, Japan had little choice but to attack Pearl Harbor. War with the US was inevitable, and it gave them a chance to get in a quick victory.
As far as Hitler declaring war on the US, it probably didn’t matter. Russia would have eaten him up anyway. Hitler was hoping the Japanese would return the favor and declare war on Russia. (as if!)