The Wildspace is the Astral Plane.
The Astral Plane includes the Astral Sea and the Wildspaces and the Dominions.
No, it isn't.
Per the Spelljammer Guide: "The Wildspace is where the Astral Plane overlaps the Material Plane."
It is the part of the Astral Plane that entangles matter. This region of the Astral Plane coheres closely with matter, even to point that Astral creatures can age there. It is like an augmented reality over the Material reality.
In Wildspace, one can visit the surface of the planet, and hang out astrally in a pub. The Astral one can see the patrons in the Material Plane, but they cant see the one in the Astral Plane. They pass thru each other. Likewise there are flying whales and all manner of Astral creatures, that coexist like virtual reality augments overlaying matter − that from the point of view of the Material Plane are unobservable.
To shift into Wildspace to interact these Astral creatures is a planeshifting.
The Wildspace is "in" the Material Plane, analogous to how the Border Ether is "in" the Material Plane.
No, it isn't. Did you just read the one sentence you think agrees with you?
Let's look at the WHOLE section here.
"
The Astral Plane is, quite literally, the plane of stars. More precisely, it is where the stars and portals to the heavens reside—an infinitely vast celestial void that surrounds all the worlds of the Material Plane.
Every D&D world—whether round, flat, or some other shape—exists in an airless void known as Wildspace. A world might be solitary, or it might have neighbors: one or more suns, worlds, moons, asteroids, comets, or other bodies. This neighborhood of celestial and planetary bodies is called a Wildspace system.
In Wildspace, the Material Plane and the Astral Plane overlap. Creatures and objects in Wildspace age normally and are effectively on both of those planes at once. If you were to leave your home world and continue outward until you neared the edge of your Wildspace system, you would begin to see a faint, silvery haze. By traveling into this haze, you pass from Wildspace into the Astral Sea, more colorfully known as the Silver Void. The deeper into the Astral Sea you travel, the thicker and brighter the haze becomes, but the stars that shine through it are always visible. Wildspace and the Astral Sea together comprise the Astral Plane.
The Astral Sea, like Wildspace, is a void; however, it is not an airless one. Here, you can breathe normally and exist indefinitely, never aging and never needing food or drink. You can propel yourself through the Astral Sea with the power of your mind alone, though many astral voyagers wisely travel in well-armed ships, for this place is the home of a host of fearsome creatures. Here travelers might find the petrified hulks of dead gods and swirling pools of color that serve as portals to other planes of existence. (For more information about color pools, see the Dungeon Master's Guide.)
Much in the way that oars and strong winds enable travel by sea on terrestrial worlds, magic items called spelljamming helms are used to propel and steer ships through Wildspace and the Astral Sea. A spelljamming helm customarily takes the form of an ornate chair in which the ship's pilot sits. To attune to a spelljamming helm, one must be a spellcaster. The pilot of a spelljamming ship is called a spelljammer."
According to the book, the Astral Plane surrounds the Material plane. And all DnD worlds exist in an airless pocket of space referred to as Wildspace, which is surrounded by the Astral Plane. You LEAVE you world, travel up into Wildspace, and then if you continue to the edge of Wildspace, you can enter the Astral Sea. And, spelljammer are like oars and wind used to propel sea ships. They are spaceships. Literally. The Astral Book gives us comets, stats for stars and suns, moons, spelljammers are spaceships. It literally says "
A typical Wildspace system has a sun plus a number of planets and moons orbiting it." later in the book. It. Is. Space.
If you want Wildspace to be an "immaterial world" then you have to accept it can be reached by just physically flying up into space... because that is EXACTLY what spelljamers DO. It literally says in the description of the item that they move through water, air, or space. There is not a single mention of planeshifting. Take a spelljammer on the surface, move it through air until it reaches wildspace, and you've entered wildspace.
If you go back to the surface? You leave Wildspace. Because Wildspace is, again defintionally, "
Wildspace systems are airless oceans teeming with space-dwelling life forms, including spores, space plankton, and larger creatures that resemble fish and aquatic mammals." Just like docking at a port means you are no longer sailing on the ocean, going onto a planet means you leave Wildspace. It even, multiple times, points out that you can use Teleportation magic to travel to different planets in Wildspace, or from Wildspace to a planet. You will never be able to "exist astrally" in Wildspace, virtual and unobservable from the material plane. That isn't how it works AT ALL.
YOU.
ARE.
WRONG.
I'm sorry to be blunt, but you are.
Ok. I was wondering if you meant something like this extra-dimensional axis. An Ethereal creature has an (instictive?) ability to shift back-and-forth between Border and Deep.
To some degree, this happens naturally. For example, when traveling in the Feywild from one locale to an other locale, the distance to there differs from the distance the back. The distortion of the reference points in the Material Plane fluctuates.
Can you think of a specific example where characters (or even a monster) shifts from Deep to Border or viceversa?
In the DMs Guide: "To reach the Deep Ethereal, one needs a Plane Shift spell or arrive by means of a Gate spell or magical portal." To shift back-and-forth between Border and Deep explicitly is planeshifting between different planes of existence.
DnD practically never uses the Ethereal for anything. That's why I want to change my personal version into the Dream Realm, because the Ethereal is empty of anything interesting to do. So no, I can't think of a time that it happened. Because nothing actually happens in the Ethereal, but in theory, it should happen, because otherwise what is the point of the two?