Proto-Greek language was formed in Southern and Central Greece after the infiltrating of IE-speakers through the northern regions of the country. While the Indo-European part of proto-Greek was formed in the region of southern Serbia, as indicated by the northern source in the Mycenaeans (Serbia_Mokrin and Serbia_EBA are the best proxies for steppe and European_HG ancestry in the Mycenaeans, this archeological culture has also plenty of Aegean outliers). The northern regions of Greece were unconquerable, there were large, well-fortified cities with non-IE population. There was no chance for a wild Indo-European population to conquer regions with such superior military power. The southern part of Greece was at the same time well-conquerable and completely unfortified.
"Also known as Macedonian matt-painted ware, north-western matt-painted ware, Doric ware or Boubousti ware (after the excavation site, now Platania near Voio, where Heurtley discovered it in 1927), pottery with matt-painted decoration is widespread in the Late Bronze Age and Early Iron Age. Most of the find-spots are concentrated in Western Macedonia (45 in the Kozani prefecture alone), especially along the river Aliakmonas, spreading into Epiros and Albania as far as Korçë and sporadically into south-western Albania (the tumuli in the Drin valley), Pelagonia, Central Macedonia as far as the river Strymon, and south into Thessaly, Elasson, and Marmariani – a dissemination which is presumably due to the constant movement of pastoral populations. Scholars of both earlier and modern times believe that the pottery was manufactured by the north-western Greek tribes, Herodotus’ ‘widely roaming nation’ (1.56). Among these tribes he includes the Macedonians and the Dorians, who, he says,
travelled from the south northwards and also settled in the Pindos mountains. The Spercheios valley is believed to have been a major halting-place in the migrations of the Macedonians and the other north-western Greek tribes; matt-painted ware of the Middle Helladic period has been found at Lianokladi near Lamia."
"The finds from Aiani finds leave no further room for doubt that
the north-western matt-painted ware was brought from the south by people returning to the north and north-west (to Aiani in the 15th century bc), after having moved south at a much earlier date or having moved back and forth owing to their pastoral economy and their nomadic lifestyle. These people were none other than the Macedonians of the historical period, whom the literary tradition directly associates with the Dorians."
Web-based project on Aegean and Balkan prehistory. Articles, bibliography database and discussion concerning the Aegean, Balkans and western Anatolia during late 3rd, 2nd and early 1st millenium BC.
www.aegeobalkanprehistory.net
The Dorians presumably dispersed into Macedonia and Epirus, assimilating local Neolithic populations, from the Spercheios valley, because the center of distribution and the earliest traces of Mycenaean Doric-type pottery have been found there (as far as I understand). Herodotus also writes about conquering Thessaly by the Locrians, and archeology echoes this opinion. The Thessalian regions are conquered in later eras by Greeks from the south, I can't find the quote yet... Most modern day archaeologists and historians believe that the Mycenaeans are the Proto-Greeks.