re: What would Hitler’s legacy be had he not been antisemitic?

He would have likely just been an unknown artist.

Hitler's drive to "reunify" German peoples (see: Lebensraum) came from what he saw as the reason Germany lost WW1 (and was thus trapped under the Treaty of Versailles): Germany being stabbed in the back by the communists and Jews.

Without that excuse of having specific groups to target with his blame for Germany's loss, he may well have never had the fire to become what we now would call a "community/political activist" and he certainly wouldn't have had an enemy to promote to the rest of Germany to fuel his rise.