I don't think Perun and Thor are related. The only thing I have to go on is the fact that Slavic is a Uralic language which isn't even Indo-European. Maybe there were some cultural ad-mixture, but any position on the topic is highly spurious. Too spurious to make a claim. But, if I were to make a claim, I'd say the similarity is convergent in nature. As for the steppe being Germanic. No, I don't think that can be bared out. The origin of the Germanic Peoples is clearly the Baltic. Any Germanic influence on the steppe would be due to migration after the fact. Talking about the Goths for instance, that's MUCH later in the story. At that point we are no longer talking about origins of a race. We are talking about conquests of a race. As for the Scythians, they were Indo-Iranian. So, any common ancestor would probably be the Yamnaya themselves. Much more distantly related than the Germans/Celts. But, yeah, the Huns kicked the hornet's nest. Things would probably be much clearer if they hadn't pressured the migration period.