The problem is that you keep insinuating that ancient Greeks were more 'northern' shifted than they actually were. You do realize they came from Anatolia, right? And, their admixture was mostly Early Farmer mixed with some Kura-Araxes. Their steppe admixture was extremely minminal, so if we actually got Indo-European language from Yamnaya they didn't effect the admixture much of Greek or Anatolian cultures that picked up IE languages much at all.
Now, modern mainlanders except for Cretans and Greek islanders have completely lost the Kura-Araxes admixture and gained more Yamnaya admixture. When do you think this event occured? I know there is a big lack of ancient Greek samples, and if we have some from even the early AD period we'd be able to confirm or deny certain theories. But I don't know why you would be ashamed to have some slight South Slavic admixture anyways. Many Greeks have Albanian and Bulgarian ancestry, why do you not like the idea of that?
About language and culture - the people who moved into Greece were not 100% Slavs. They were our northern neighbours who already had some Slavic admixture, AKA south Slavs. You seem to think they are the same as a Ukrainian or something, but they're not. The admixture is similar to ours but lacking the Kura-Araxes as I showed before, now you need to stop coming up with conspiracy theories - mainland Greece was never isolated like Iceland so I'm not sure why you're making a comparison.
Honest question here, do you think Greek admixture remained the same during these events?
https://i.imgur.com/EYf0Gfv.gif
https://i.imgur.com/4gxJdOh.gif
https://i.imgur.com/V19gJzO.gif
I doubt it, but I'm open to hear why you don't believe admixture did not change when there is so much evidence it did.