Question is : Whether it ever existed any Slavic tribe, independently from Germanics, Scythians, Sarmatians, to have their own independent origin, wihtout the influence of other speakers (at the begginings, of course).
Short answer: No.
Slavic comes from Balto-Slavic and the transition didn’t happen overnight. The 2 language groups were separating (and are still separating) from each other for thousands of years. Throughout all those centuries Slavs and Balts came into contact with many tribes of various origin and that means speakers of other languages. If multiple ethnic groups lived among each other, they surely exchanged some portion of their languages. Especially if the languages weren’t written. That was the case of Proto-Slavic.
Maybe Slavic and Baltic separated from each other because of influences from other languages.