Or maybe the Proto-Slavic [or at least Proto-North Slavic?] homeland was in Slovakia, Upper Hungary and Zakarpattia? According to a Medieval Legend from my region (Wielkopolska), Slavs originated from the Carpathian Basin and their ancestor was called Pan (the name of Pannonia comes from him):
https://en.wikipedia.org/wiki/Lech,_Czech,_and_Rus
"According to Wielkopolska Chronicle (13th century), Slavs are descendants of Pan, a Pannonian prince. He had three sons - Lech (the youngest), Rus, and Čech (the oldest), who decided to settle west, north, and east." - Lech was the ancestor of Poles/Lechites, Rus was the ancestor of East Slavs, and Cech was the ancestor of Southern West Slavs.