Your ground path is Ok cuz all your manipulatin has been on the power side. The symptoms ain't adding up cuz jumping the 2 lugs on the starter is the same as jumping the 2 lugs on the starter relay (assuming the small "solenoid" wire from relay to starter has continuity. I'd clean the ring terminals on the 2 big wires on the large "batt" terminal & that threaded stud on the starter relay and check/clean the smaller solenoid wire/terminal at the relay. Starter is good/the batt is charged, just a loose/poor connection in that circuit