In my own game, there reverse is true. The inner planes preexisted the outer planes. Indeed, while they are called planes, the outer planes aren't really planes but dominions - places carved out of the astral plane by the gods after they departed the material plane. The inner planes including the elemental planes, the ethereal, and the astral were all in existence before the gods. The inhabitants of the outer planes are mostly creations of the gods - for the most part, the greater servitors. The two exceptions are the Modrons, who claim to have come into existence when the world was made, and the Slaad whom no one is quite sure where they came from (Ygorl knows and perhaps some of the other Slaad lords and possibly Primus of the Modron, but they have their own reasons for not telling anyone). Other than the Modron, the immortal fairy races are the oldest races in the multiverse - which is why the formal mode of address to a fairy is "Eldest and Youngest". The fey trace their existence to the pollen that fell and still falls from the flowers of the tree of life. The animals and plants (and the drakes and the magical beasts) trace their existence to the animating principle of life which radiates from the tree, and they are represented collectively by the spirits of the fauna and flora. The lords of the fey that were sprung from the elemental planes became the genie.
Sometime after this the gods came into existence. They settled the jewel of creation and started having families, with each other and with the genie. The offspring of the gods and genie became the giants, and with each other the lesser gods and titans. As new fruit on the tree ripened, new gods joined the scene. At some point, a dispute rose up amongst them over who was greatest and who should rule. In the aftermath of this dispute, the spiritual sun of Uman, self-proclaimed "Lord of Creation", by the name of Usurl usurped his father's claim to the throne by committing the first murder and proclaimed himself Death - the new Lord of Creation. This act led the first God's War, which resulted in the shattering of the material plane and the permanent death of many of the gods. The gods would eventually declare a truce and one of the terms of the truce is that they would depart from the material plane before it was completely destroyed, so they each took up abodes in the Astral Plane and built dominions there. Those dominions are what morals call "The Outer Planes", but they are really demi-dimensions. That these dominions are built more or less adjacent to each other is what causes mortals to imagine them as a "Great Wheel", but this is just in reality an example of mortals 2D thinking.
One of the other terms of the treaty is that the gods would collectively create a new lesser servitor race modelled after the fey that would be tasked with repairing the material plane. But the treaty negotiations almost broke down over what form this servitor race would take, as each of the major families of deities insisted on a design that reflected in miniature their own traits. It was eventually decided that six designs would be submitted, on the condition that each design be such that it was given the dignity and right to choose to worship and serve whomever it liked. To make sure these new beings would never represent a threat, they were made mortal like the animals and given only weak and seemingly minor gifts. Indeed, the surviving children of Uman, still in grief and wounded by the great losses of their family in the war they had started, paid hardly any attention to the problem at all and made a wholly generic design seemingly without gift at all. These independent beings became collectively known as "The Free Peoples" - although in practice that term is often extended to philosophically include the fey that were the inspiration for the concept.
But, and this might not be immediately obvious unless you think about it, something went very wrong. The recognition that something is very wrong with that orthodox account is the basis of for most of the cosmic mystery of the setting as well as the various heresies that crop up. There are things even the gods don't understand and there are things that they do understand that they do not talk about.
One big difference between this setting and the default D&D setting is at his zero infinities in it. None of the planes are infinitely large. There are not an infinite number of dimensions. There is no infinite time or space in it. Things are enormously big - I often compare the size of the Astral Plane to the size of the real solar system in that if you moved at mere walking speed it would take you hundreds of thousands of years to traverse it - but nothing is infinitely big. There are countable numbers of everything.