I'll repost my theory:
"I personally imagine pre-L51 and pre-Z2103 splitting somewhere in the Balkans, or maybe Anatolia, with pre-L51 travelling (perhaps by sea) to Iberia and pre-Z2103 spreading somewhat Eastwards across West Asia. L51, part of the Iberian BBs and amongst typical Megalithic folk, would then travel to Central Europe (acquiring some more Steppe-like mtDNA lineages from Corded Ware women along the way), before expanding throughout Western Europe as part of the Unetice cultural complex. Z2103 would have both remained in West Asia, but also moved up into the Steppe, and from those people Yamnaya would expand into the Balkans. This entire process would be at least at first associated with the spread of metal (L23)."
Any points of contention are welcome. There's a few key reasons why I believe this over the Steppe origin of L51 though, just for example the fact that there doesn't seem to be any cultures to find L51 in, with the assumption that it would have left a trace in the present-day location of its mother culture, assuming a Steppe origin. L51 is pretty much entirely confined to Western Europe, and the Balkan expansion of Yamnaya seems so clearly linked to the present day distribution of Z2103. Moreover, a lot of the earliest subclades of L51 are in Sardinia of all places, which points to this maritime theory strongly.
L51 could have picked up Steppe admix from Corded folk, but it could easily have had Steppe admix to start with. Low noise of Steppe admix has been found in the Iberian Chalcolithic, and in this theory the carriers of this admix would have likely expanded from the Balkans, where there has been (obviously ignoring the Danubian farmer samples) a Steppe presence for a long time. I personally believe that the Balkan-Black Sea region is the original breeding ground of R1b, meaning a large Steppe presence among non-farmer samples (who were clear imports from the Neolithic Middle East, bred like rabbits, and can be paid little attention in this hypothesis) is to be expected, and has been found already (as one example, a Greek Neolithic sample has been found that even clusters with individuals of Northern European Corded Ware origin - Neolithic!).