I like the idea here. What if we made it a bit less monolithic?
Instead of the Aasimar being a horde of invaders, what if the Aasimar civilization existed. The expansion into the territory of the elemental princes isn't core to the Aasimar civilization, just a something done for strategic reasons.
Instead of an all-out assault, they are exploiting the division between the elemental princes, ruling the elemental peoples via proxy, etc.
You can even go into the dawn war, and have the Divine forces create the mortal races by manipulating the Elemental peoples. The mortal races would act mostly as background civilians and laborers in the conflict, relatively weak compared to the divine powered Aasimar and the Elemental powered peoples.
The Divine corruption of Dragons produces the Metallics, that is awesome. But the Divine corruption of Giants produces Dwarves, for example; a smaller, easier to control, people who can do the crafting that the Giants where doing.
We can even define a few areas of conflict.
Astral Sea + the Gods control the high ground. The Astral Sea permits relatively easy travel, and the Upper Plane sanctums are fortified spots from which their forces spread over the cosmos.
Prime Elemental
This is a land of raw elemental beauty. The raw force of creation is everywhere. Travel in the Prime Elemental is slow, and much of it is pristine wilderness. Different elements dominate in different areas. The great and ancient Elemental cities are here.
The Three Kindgoms
The great ring is a circular flow of soul energy, from which the Elemental Princes crafted the core of the Elemental Peoples. Properly crafted and buffered with elements, a soul is self-sustaining and can reproduce, so while the great ring was needed to start it off, it is only needed to craft a new people.
When the divines arrived, they wanted in on the game. They set up to harvest the great ring, and built a dam in it. This divided the ring into three kingdoms; Fae, Fel and the Middle Kingdom.
This was done with the cooperation of some of the Elemental Princes, as they to could see the benefit of being more easily able to create new people's from the great ring. Others objected, but everyone had more important things to do.
From energy captured in the middle kingdom, the Divines crafted Angels out of nearly pure soul energy. These creatures where not properly buffered with elements, and found existing outside the astral sea difficult. The gods, lacking the ability to manipulate the elements like the princes, purchased Elemental peoples and replaced the souls of newborns to produce Aasimar.
Each one has an Astral soul that does cannot reproduce, but does reincarnate. When the reincarnation happens, the Aasimar is recreated an adult.
It was known the divines paid well for captured elemental people's, but the horrifying soul replacement wasn't known or believed by most.
So now, we have Divine fortresses on the Great Ring, harvesting soul-energy while trading in elemental people slaves. Using astral magic, they are warping the elemental people's to produce more ameniable slaves, such as shrinking Giants into smaller and less arrogant crafters, but that effort is just beginning.
From the fortresses in the Middle Kingdom, the Divine forces have advanced into the Prime Elemental, and are claiming territory. Aasimar advisors back one side of the other conflicts between the princes; with access to the Astral Sea, elemental forces can be moved over the Prime much faster than they could without Divine assistance, which means that Princes that ally with the Gods have a significant advantage.
Over time this has resulted in Princes becoming subject to said Divine forces. Those who don't bend knee are cut off from access to the Astral, which becomes strategically crippling.
