We have an Iron Age sample from Iran:
What I find interesting (in comparing the two graphs - whoever edited them) is the parallel genetic flows : one in northern Europe, developing homogeneously from the steppe, and one in southern Europe, with a clear CHG/Iranian influx.
As I see things, the pink dots labelled Thessaloniki are what became of the populations Pip refers to as East Balkan (Ezero derived),
after they had received a slight influx of CHG. As one moves south from them towards Mycenaeans, the CHG+EEF regularly increases, and the steppe gets increasingly diluted (with the exception of Crete, pulled northwards by Dorians et alia).
The more I think about it, the more convinced I am that BB steppe ancestry didn't come through Romania/Bulgaria. Or if it did, it didn't linger there. I even wonder whether L51 came via Hungary and across the Carpathians at all. They didn't have as much CHG as the steppe people just north of the Caucasus. It is easy enough to assume they moved from some point further north on the steppe (under threat of R1a tribes?), and went west skirting CWC territory, then along the northern slopes of the Carpathians, into GAC territory, or what was left of it. Bringing their own version of Centum PIE with them.
I honestly did consider Suvorovo and Ezero as source pops for BB at one time. But Suvorovo is definitely too old (and had vanished long before BB emerged), and Ezero would have been too admixed by 2500 BC, after 600 years of rubbing elbows with farmers.
As for PIE originating south of the Caucasus, I don't know any better than the next guy, but I find it strange that those R1a People, up north in the forest steppe, should have taken to a geographically remote language just to please their neighbors and trade with them. People change languages when they are militarily, or economically, or culturally, superseded. It takes time and pressure. Gaulish is reported by Sidonius Appolinaris to still have been spoken in rural Gaul more than 500 years after the Roman conquest, in spite of Roman cultural prestige and economic prowess.