if the collapse never happened?
That's the hardest part. The "barbarian" peoples on the move to fight the "civilized" countries seem to be the inevitability at that period, whatever the reasons of their displacement had been.
But there were "barbarian" migrations, which were stopped: the Celtic army was defeated in Anatolia by the Celeucid army and their war elephants. The Galatians had to settle there; being a threat and nuisance they stopped being an apocalypse.
The Cuimri and Teutones were just annihilated by the Roman legionaries of Marius, there was none of the "Barbarians" left to settle; for years to come on the former field of battle the Roman peasants made fences out of innumerable white bones of these unhappy Barbarian invaders.
So in order to stop "apocalypse" we need some POD to create a great Empire covering the area of the awaited invasion; well, at least Anatolia and Syria united by a chrismatic leader is doable. A few dozens of war elephants would be handy.
The Mycenians are pretty doomed (sorry, guys).
Egypt is likely to fight off what is left of the "Barbarians"; probably no "Sea peoples" would reach Egypt as they seemed to use coastal navigation, they could not cross open sea of the Mediterranean.
After the "invading barbarians" are properly humiliated, annihilated and partly settled, the "Charismatic leader" who saved the civilized world dies, gets deified and his Empire is desintegrated into what it had been at the time before invasion.
And we can calmly speculate on the subject: "What if no bronze-age dark ages?"