Submersible pumps don’t usually wear out gradually unless you are pumping abrasives like sand or silt. They usually run close to 100% capacity until they quit running all together. Gradual reduction in flow is usually from a plugged screen on the pump, well, or both. My guess is the well screen is plugging up. The well is now only making 7.9 GPM, the pump is trying to pump 11.9 GPM, it is sucking air, losing prime, getting hot, and that’s it.
Get a new box, and check the well production rate.