Morkus from Orkus
I get that.
For example, the Astral Plane overlaps the Material Plane. But each is distinct from the other.
(Indeed it is planar properties of the Material Plane that cause Astral creatures in the Astral Plane to "age". Thus Astral creatures can only have children who grow up while in the Material Plane. Afterward, the family returns to the Astral Sea where they can no longer age.)
If it's your game, just make it so. Nothing awkward about it.My preference is: Astral dominions are part of the Astral Plane, but arent part of the Astral Sea.
Unfortunately, a single sentence in a sidebar in Spelljammer makes this awkward (Guide 21).
"Because these dominions are part of the Astral Sea, they are timeless. Nothing ages there, and creatures can survive there indefinitely without food or drink."
Those astral dominions, though, are not the outer planes. They are the city of brass, some god homes, etc. There's no conflict between that sidebar and running the planes as distinct from the astral sea.While it is true that Astral dominions are (typically) ageless, a dominion can have unique planar properties to itself.
For example, the dominion of Hestavar is the concept − the paradigmatic thought construct − of the ideal civilization. I expect and hope it will have unique planar properties that cohere with this archetype. (Star Trek tropes might make sense here.) In some ways it already has properties, like gravity, which the Astral Sea lacks.
I expect the same form the Outer Planes, aka alignment domains. Each will have unique characteristics.
Even tho the domains are part of the Astral Sea, each can have specific characteristics that are unlike the rest of the Astral Sea generally.