While J2b-PH502>Y40852 branched from its ancestor at some point during the Iron Age (c. 5th century BCE), the MRCA was born during Late Antiquity (c. 3rd century CE). This leads me to believe that this lineage expanded southwards during the Roman imperial period (consolidating somewhere between Herzegovina and Kosova), and that it represents a native Illyrian branch that was consequently Romanised prior to the Slavic migrations.
The Erzyas you mentioned form a medieval branch under J2b-PH502>FT195999, with the TMRCA dating to the 11th century CE. Considering how broad the formation-TMRCA gap is (c. 475 BCE-1075 CE) and the fact that the parallel branch is found in the Balkans with a Late Antique TMRCA, I think its most certain that J2b-FT195999 expanded northwards from the Balkans during the Middle Ages and was not a branch associated with the early Slavs.
As for the issue of ethno-linguistic identity, the current data doesn’t really allow us to determine for how long your ancestors have been Serbs or for how long the Albanian sample’s ancestors have identified as Albanians; although I imagine it has been for a good number of centuries. I see that the Albanian and Serb from Raška share an MRCA who lived during the 11th century, I think its quite possible that they both descend from Vlachs that lived between SW Serbia and Kosova.
