I would be VERY careful about adding sand to the ring, because getting the surface and under-layers too loose, deep, is not in the best interests of the horses you ride there.
Horse should have not more than 2 inches of soft, loose stuff to sink into, before his hoof hits firm surface to push off from.
Going soft and deeper than 2 inches, makes legs work much harder and fatigue faster for soft tissue injuries.
Granted your ring is older, but if you did the chain harrow dragging more often, would it keep the surface a bit looser and softer, but not deep? Lets you keep the firm surface, but not hard with packed stone.
Yeah, dragging more often is more work, but if the ring has been good all these years, no other issues, I would not be adding sand to my ring.
Horse is MADE to go on surface he can sink into a LITTLE BIT, then hit hard dirt to push off of for the stride. No firm surface under, means horse works MUCH harder, legs can get hurt easier. Would YOU rather run on the dry, deep beach sand or the hard edge along the water? Which surface lets you go easier, longer? Not easy running or WALKING on deep, giving sand. Very tiring.