If correct, and Yamnayan/Poltavkan Z2103 samples were all KMS67, with other branches of Z2109 being West/Central European, then this would (i) place formational Z2109 in a position more likely in West/Central Europe or somewhere intermediate between the two, and (ii) provide additional indication of the decline of Yamnayan lineages - as KMS67 appears to have a pretty small modern population.
We cannot tell if Lentz was paternally Yamnaya-derived or from another population ancestral to Yamnaya. His STRs are only similar to two other North West European samples, and are very different to Eastern KMS67 samples, so I suspect the ancestors of these three might have broken off westwards pre-Yamnaya.
This just leaves, as you identify, two or three modern KMS77 samples, with probable early branchings into (i) Iraq, and (ii) Bashkortostan. Additionally, STRs identical to Bashkir samples show up in Dagestan, and it is not clear whether their DNA originates in the Urals or the Caucasus, although their closest relation being Iraqi would lean me towards the Caucasus, particularly as other basal clades of Z2103 also coalesce there.
As is the case with autosomal data, this does suggest there was a shrivelling of Yamnayan DNA, and its replacement by R1a lineages and Corded Ware-like autosomal DNA does suggest a population change in the region, rather than an evacuation of it, would you agree?