We used an 8 hour car and driver in the last week of May, which worked out fine for us. 1 1/2 hours free time each in Positano and in Ravello, less in Amalfi (which I found a little underwhelming anyway), and touchdowns for views in two other towns. We should have declined the suggested Green Grotto, but we knew enough to say, "Does the pottery store have bathrooms?" when offered that; We used the multiple rest rooms and moved on. Good value if expensive. We had normal, i.e. annoying but not stationary, traffic. I suspect October is better than the last week of May. (No personal experience in Oct.)
Our car service came from out (Ambasciatori, big) hotel's land services partner, charged to our hotel bill.
There's nothing wrong with ferries, but make sure you have the timetable for the date you are traveling; remember that ferry company #1 will not tell you that you could leave an hour sooner on Ferry Company #2. Also note that places like Positano cannot take as large a ferryboat as, say Sorrento. That means less capacity and more wave action.
You should note that the Capri and Rome travel is not negligible. This is a very busy day. And the boat ride to Positano is longer than to Sorrento. Many Capri hotels are some distance/time from the ferry docks. Make sure you learn where along the 1/4 mile of docks your actual boat is leaving from. Your daylight time (check astronomical info) on the AC will be limited. You may wish to prioritize, fewer. destinations. Note that a discounted Frecciarossa train ticket cannot be used on the next train if you miss your departure. You have to buy a new one at the walkup price.
I'm not trying to talk you out of it, only to keep your eyes open to your plans. I'm sure Positano is lovely from the water. But I was satisfied with the views from the beach and from adjacent highway viewpoints.
