Before pulling the starter motor it’s worth checking the starter solenoid/relay. I had similar problems about 5 or 6 years ago with it ending up not wanting to turn over at all. I shorted the solenoid out with a screwdriver and he spun over straight away.
I priced up a genuine part and it was stupid money so I got a Fleabay special for about £10 or £12 and haven’t had a moments trouble with it since.
It’s possible that one of the 2 bolts through the cables have come loose, but because it’s rubber mounted it’s unlikely, but there could be some corrosion there offering a high resistance, worth checking the battery terminals are tight too