As I explained in
post #55 above, modern Greeks, and particularly northern Greeks, are quite different from Minoans and Mycenaeans. Don't be deceived by the simple admixtures using ENF, CHG, EHG and the like. I estimated that to increase the EHG from 7% to 20%, it actually requires the contribution of 25 to 40% of non-Greek European DNA, depending on the source populations. Based on modern Y-DNA in Greece, it can be deduced that the Slavs contributed the most (21% of Y-DNA in modern Greece), followed by the Germanics (10%) then the Romans and La Tène Celts (8% together). That's 39% on the Y-DNA side, but overall it's likely to be a bit less than that as the paternal line of invaders tends to outweigh the maternal line. It's probably less the case for the Slavs and Goths, who moved as whole families, and indeed whole tribes, but it would be truer for the Romans, who were mostly administrators and soldiers stationed in Greece, with few Roman women settling there.
In summary, it's true that the impact of the Romans on these populations was relatively minor (1 to 5%), but that is to be expected as the Romans did not send a big number of colonists to places like Egypt, Phoenicia or Britain. The places most heavily colonised by the Romans outside Italy were Gaul and Iberia, particularly the southern parts like Provence and Andalusia. It would be much more interesting to see the population shift before and after Roman times in those regions.