If you don't want to be a part of the D&D Multiverse, you can't play D&D 5E.![]()
I know you added a smiley, so I am cool with your intent, however...
While the majority of my campaigns are set in the "multiverse", two in particular are not.
A. The Million Spheres; millions and millions of alternate worlds connected to each other via proximity or paths. There is no "in between", no Astral, no Outer Planes, its all worlds (until you hit turtles of course..

B. A spin off of Zelanzy's cosmos. One Prime world, everything else shadows until the shadows become chaos. Again, no "in between", no Astral, no Outer Planes.
Both still D&D 5E. Certainly, you could describe both as sitting off in the corner of the D&D cosmos, should you wish to be labor the point.

That of course would require appointing a second and pistols/swords at dawn, your choice of course. I am the epitome of politeness.