can you try to create a basic red sphere and just add it to your main group that’s housing all the child models? I’m carious to see where your group’s pivot point is.
When you add a 3d object to a group or empty mesh, the child automatically gets positioned at the parent’s pivot location. So without modifying any position on the sphere, where the sphere appears in the scene is where the group’s pivot point is (a.k.a. the group’s position)
In fact, after you add the sphere, and if you rotate the group, you will see that the sphere should stay in the same location since the sphere is at the group’s pivot location.