to add on this, slavs might not be a single ethnicity per se, but they are certainly a genetic cluster from north-east europe, modern day belarus or the baltics. west slavs, east slavs and baltics are very close genetically so it's safe to assume that they were close as well to the original slavic cluster that came into balkans. the reason we cluster together with serbs and bulgarians is because we're somewhere halfway between slavs and greeks , meaning that we're a mixture of these two. by greeks i don't necessarily mean the modern day greeks, but the actual byzantines which contributed to the romanian genome as well.
Romania