Obviously the enabling law allowed Hitler to rule by decree and was regularly reauthorized so that technically the Weimar Constitution never went out of force. Did the Nazis go as far as to pass laws/decrees that authorized the actual killing of the Jews?
As you've observed, the Holocaust had no legal grounding under German law at the time, and thus yes, it was illegal. u/commiespaceinvader covers said illegality here and also expands further here.
And of course, should anyone want to offer their own input, more posts are always welcome.