When turning over once or twice, is it fast'ish or slow? Barely rotating? or fast then stop? If battery is not breaking down during load, which you can't tell unless a load test is done, then it does sound like a starter problem. You can use a 12V battery to see what happens. It won't hurt the starter if you don't lay on the pedal too long. That may identify a bad battery or a bad starter. I don't think you have a ground problem if you cleaned where the cable attaches to food pedal bracket or to starter bolt. Where is it connected now? For turning over motor, there is only one ground that matters, the negative cable attachment. I assume you have checked the positive cable clamp and battery post and the negative clamp and battery post and have shinned up everything and tightened.
WARNING Advise us BEFORE you do the 12V battery.
