To what level were the Slavs considered "Untermensch" in Nazi racial policy?

I've been trying to find primary sources on this, and I would appreciate any help. I've encountered some strange theories from obvious closet neo-nazis, which is the first huge red flag, where it is said how the Slavs were never seen as subhuman, and were part of the "six races" established with the Nuremberg Laws in 1935 as Aryan and German-worthy. This to me seems awfully contradictory considering the most amount of people killed by the Germans were Slavic. I did find some documents pertaining to be against Soviet Slavs, which makes we wonder, what was their view on Serbians? and did they really think the Ustashe were that bad? I would assume not considering they didn't do anything about them, but I would appreciate if someone could explain this better to me as I'm tired of neo-nazi propaganda.