It's a fact of the paragraph you are referencing in the Spelljammer book.
From the sidebar you have been quoting, the very first thing it says...
"Many gods have dominions in the Astral Sea. These locations typically take the form of floating islands or cities of fantastic proportions. Astral travelers might visit these dominions as they would any other ports of call, though a dominion's divine ruler always knows when visitors have arrived and what their intentions are."
That's what astral dominions are in 5e. They are not the outer planes.
No. It's a facet of the outer planes which are ALSO built around thought. However, unlike the astral, they are thought and spirituality both, given physical form.
No it's not. It's a plane. Gods can have dominions on one, but the plane itself is not a dominion.
That's a great cosmology for you to create and if I played in a game with that sort of homebrew, I'd think it was cool. It's not the default of 5e, though, and Spelljammer doesn't change that at all.
They can do that anyway. They just need to find a big enough color pool to fly through.