First of all, I've read so many possible genesis for the Slavic people, I'm not even sure which one we should consider...
Are there episodes of recorded slavic contacts with the Sauromatae? Afaik Bastarnae and keltic groups took over the western Ukraine, forcing the Slavs to fall back further north (they would return later in the 6th century AD)...
And as I said before, of archaeological evidences that I know of further north-west, we have only scythian hoards dating several centuries before the EB start date...