It seems like lots and lots of Greeks moved to Persia, the Levant and Egypt at this time and I’ve also noticed that Macedonia and the rest of Greece seems to have been the weakest element in the Hellenistic world, basically reverting to it’s extent under King Phillip II and keeping none of Alexander’s conquests and being the first to fall under Roman domination, I wonder if this is because mass emigration from Macedonia and the rest of Greece to Asia or Egypt caused depopulation, and thus they had fewer men to recruit as soldiers, fewer farmers to feed an army, less tax revenue with which to fund a war or hire mercenaries etc?
This is an interesting question and I spent some time yesterday evening having a look around, but I did not find satisfactory information for a true answer. There was unquestionably migration out of the Aegean-adjacent areas into Egypt and the Near East, but the sense I get from the sources is that most of these "colonists" were Macedonian in origin, which actually means those who would be considered "Greek" at the time (mostly from coastal enclaves) and also those from the Macedonian interior which would not have been considered "Greek" 100 years previous. In fact, there is a general expansion of what it means to be "Greek" in the Hellenistic period, with many decidedly non-"Greek" peoples all over the Mediterranean increasingly adopting modes of life which we identify as "Greek" or "Hellenistic" in the historical and archaeological record. The construction of a gymnasium or a bouleterion in a town on the Nile does not necessarily require a fully "Greek" clientele, or even "Greek" builders. This contributes to a picture we get of an explosion of "Greeks" spreading out from Spain to Afghanistan, but the reality is probably more nuanced: certainly migration, but also a general widespread adoption of Hellenistic modes by huge swathes of non-Greek populations.
I can't find any satisfactory evidence about the consequences of migration back home in the Greek heartland and Macedonia in the Successor States period. Your observation about Macedonia proper after Alexander is correct: it became the lesser of the kingdoms, according to the typical metrics we modern historians use to assess such things. This is not so much due to population movement, or brain drain, I don't think, but more about the realignment of polities and the expanding definition of statehood. Greek city-states of the 5th century were, by their very nature, quite small on average. There were outliers, like Athens and Syracuse, but the typical city-state had a citizen population counted in the tens of thousands at the most. When Athens came to dominate her "allies" in the lead-up to the Peloponnesian War, she was embarking upon an experiment in transformative state formation: she was becoming bigger than any traditional polis, Ian Morris' "Greater Athenian State." Likewise, when Macedon entered the scene, Philip II's reforms had facilitated a state organization, under a monarchy, which harnessed a much greater base of resources and manpower than any one city-state could muster, even Athens or Sparta. The same is true for the Successor States. They were just bigger, in every conceivable way, than the polities of the pre-Macedonian Greek sphere. The amount of human beings and raw materials under the single rule of the Seleucids was simply enormous, vastly more than Macedonia or even all of "Greece" combined. The stakes had gotten much larger.
In looking for information on this question, I consulted, without much success:
Austin, The Hellenistic World from Alexander to the Roman Conquest
Burstein 1985, The Hellenistic Age from the battle of Ipsos to the death of Kleopatra VII
Carol King 2017, Ancient Macedonia
Erskine 2005, Companion to the Hellenistic World
Peter Green, Alexander the Great and the Hellenistic Age
...and a few sourcebooks.
I'd be interested to hear from others on this question. There is a lot of epigraphic evidence, especially from Greece and Asia Minor.