The easy one to fix is the belt. In edit mode select the belt and buckle. Then separate the selection§ That will make a new object out of the belt. Go back to object mode, select the body, then shift select the belt, and go into weight paint mode. Select the transfer weights tools, and set it’s options to Nearest Edge Interpolated and source layers to by name. You can then join(J) the two parts back together.
The collar is more difficult to fix easily. You can do the same trick to it, but you’ll see that the belt still deforms kind of wonky, and the collar is way worse. The only real fix is properly painting the vertex weights. Using automatic weights very seldom gives perfect results, and almost never gives decent results with a mesh that is several different pieces.