Is there a way to create a demiplane in the astral plane and not break astral projection when entering the demiplane?

Question

A wizard character wants to create a faux afterlife for a large group of other people.

His plan to do so is as follows:

1. Cast Astral Projection to travel to the astral plane with eight other people
2. Cast Demiplane to create a doorway to a demiplane
3. Make the others enter the demiplane without breaking Astral Projection
4. Cast dispel magic on himself only (removing the Astral Projection)
5. Repeat, accessing the same demiplane each time until all people are in the astral plane
6. Store their suspended bodies for safekeeping in the prime material plane

Does a demiplane created in point 2 count as another plane for breaking of Astral Projection? And if yes, can you create and enter a demiplane in a way that won't break the spell?

Answer 1

There are problems

Cast Demiplane to create a doorway to a demiplane

You can cast demiplane while you are in the astral plane. Doing so creates a doorway on a flat and solid surface¹. Such a surface isn't something readily available in the astral plane², so you have to create such a surface, or you have to travel to a place that has one.

Make the others enter the demiplane without breaking Astral Projection

The demiplane itself is not in the astral plane. When an astral projection enters a demiplane its real body arrives as well. The astral projection choose whether to enter the body (and end the spell) or to persist as an astral projection³.

Cast dispel magic on himself only (removing the Astral Projection)

This returns you to your body, which is in the last plane you entered. In other words, if you enetered the demiplane during step 3, that's were you will return to.

Repeat, accessing the same demiplane each time until all people are in the astral plane

I'm not sure what kind of faux afterlife the you are trying to create, but the demiplane created when you cast demiplane is 30-foot cube⁴ and that's gonna get crowded.

Store their suspended bodies for safekeeping in the prime material plane

Step 3 brought the bodies into the demiplane. Assuming that the astral projections did not choose to re-enter their bodies, you can return later to collect the bodies and store them in the material plane.

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That said I am really struggling to understand the desired outcome. A room crowded in astral bodies is hardly a faux afterlife. Moreover, if an astral projection drops to 0 hp it will return to its body⁵ that you stored in the material plane, which means that any of the people in this demiplane can leave whenever they want.

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1. You create a shadowy door on a flat solid surface that you can see within range.
2. The Astral Plane is the realm of thought and dream, where visitors travel as disembodied souls to reach the planes of the divine and demonic. It is a great, silvery sea, the same above and below, with swirling wisps of white and gray streaking among motes of light resembling distant stars. Erratic whirlpools of color flicker in midair like spinning coins. Occasional bits of solid matter can be found here, but most of the Astral Plane is an endless, open domain.
3. If you enter a new plane or return to the plane you were on when casting this spell, your body and possessions are transported along the silver cord, allowing you to re-enter your body as you enter the new plane.
4. When opened, the door leads to a demiplane that appears to be an empty room 30 feet in each dimension, made of wood or stone.
5. If a creature's original body or its astral form drops to 0 hit points, the spell ends for that creature. If the spell ends and the silver cord is intact, the cord pulls the creature's astral form back to its body, ending its state of suspended animation.

Answer 2

Short answer: Yes, it is possible

So you have several questions here. In regards to your primary question, here is some relevant rules text:

If you enter a new plane or return to the plane you were on when casting this spell, your body and possessions are transported along the silver cord, allowing you to re-enter your body as you enter the new plane.

It is notable that it does not force you to reenter your body, only that it pulls your body to the new plane when you pass through the portal, and gives you the ability to enter it.

The spell ends for you and your companions when you use your action to dismiss it. When the spell ends, the affected creature returns to its physical body, and it awakens. The spell might also end early for you or one of your companions. A successful dispel magic spell used against an astral or physical body ends the spell for that creature. If a creature's original body or its astral form drops to 0 hit points, the spell ends for that creature. If the spell ends and the silver cord is intact, the cord pulls the creature's astral form back to its body, ending its state of suspended animation.

From this, we can see that the spell has no set duration, and instead lasts until you (the caster) feel like dismissing it, something causes the creature to be forced back to its body (either its astral form or its original body dropping to 0 HP), or the spell is dispelled via an outside source (targeted with dispel magic).

If you are returned to your body prematurely, your companions remain in their astral forms and must find their own way back to their bodies, usually by dropping to 0 hit points.

And from this, we know that even if the spell ends for you, it will not end for any of your companions until they purposefully reenter their bodies or are forced back in some manner (loss of hitpoints or another dispel).