I think you (we?) should first answer the question: What does it mean to be Slavic or Germanic etc? You know, in scientific literature, especially on topics as convoluted in their definitions as this, the authors usually start precisely with defining the terms they’re going to use and what do they mean exactly (in the context of the study). This is true not just for Slavs vs. Germanics, but many other similar cases as well – f.e. when we say the Cumans were Turkic, should we expect them to look like Genghis Khan (i.e. genetics) or are they classified that way simply because they spoke such a language? Same here – are South Slavs really Slavic? Our languages are Slavic, but our genes are… kinda different… So, is being Slavic more of a genetic thing or a linguistic thing for you?