I, too, am confused about your proposed timing in Florence and Tuscany.
If you are spending 2 nights in Venice, you do not have 2 days there. You have one full day plus some hours on the day you arrive, when you may well be severely sleep-deprived and jetlagged. You may be luckier, but it is all I can do to remain vertical and awake on my arrival day. I would want at least 3 nights in Venice.
Similarly, you are not in Italy "through Oct 3rd". If your flight departs at 3:45 PM you'll need to be at the airport no later than 1:45 PM, right? Or possibly earlier if the flight is not to an EU destination. So that morning you must get up, pack, have breakfast and possibly also some sort of lunch, and then travel to the airport. How much sightseeing time does that leave on your last day?
So I count this as 11 full days (12 nights)--and that's assuming September 21 is the day you arrive in Italy rather than the day you depart from the US. During that time you plan to stay in six different hotels, so that's five hotel changes. Those are more time-consuming than you probably expect. Your in-transit stop with the car may not be so bad, but otherwise I figure it's a minimum of a half-day gone every time you switch cities and hotels, so changing hotels five times turns your 11-day trip into a 9-day trip.
I think you have too many locations for 11 days. Those are world-class destinations deserving of more than blitz visits.