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.
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.
- You create a shadowy door on a flat solid surface that you can see within range.
- 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.
- 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.
- 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.
- 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.