Welcome to the forum!
First question- are you up to traveling on your arrival day?
If you could that would eliminate the one nighter/split stay in Rome- put all your Rome nights at end of trip.
Can you fly IN to Venice, out of Rome? Or vice versa? Or out of Naples?
That would open up a lot of itinerary possibilities. This is what's called a multi-city ticket (not 2 one ways)
Siena- I think it is worth 2 nights but don't see where in your itinerary to take from, so one is fine.
It makes sense to stop there on your way to Florence. From Siena to get to Venice you'll have to go back to Florence anyway to catch the train.
There is a bus from Rome to Siena that will be your best tranportation as it is direct (Flixbus I think.
(Otherwise same as above- train to Florence then bus to Siena)
Sorrento/AC is absolutely worth the time- actually 4 nights if you could- it takes a lot of time and hassle to get there- make it worthwhile. The train from Venice to Naples is almost 6 hours then you have to transfer to local train to Sorrento so that first day is mostly travel.
Have you considered staying in Salerno?- it is actually ON the AC whereas Sorrento is not. Direct fast train from Venice and to return to Rome. Less expensive lodging, less touristy, great restauarnts, access to Paestum, easy access to Amalfi, Positano, Ravello by ferry or bus. (Capri is a bit far- substitute Ravello)
So -if you want to travel onwards (I am guessing not because you are coming from west coast)
Arrive
Bus to Siena 1 night
Bus to Florence 3 nights
Train to Venice 3 nights
Train to Sorrento (Or Salerno) 4 nights
Train to Rome 3 nights
Another option
Train to Florence on arrival- 4 nights with 2 day trips Pisa/Lucca and Siena
Train to Venice 3 nights this might not work with Regatta?
Train to Sorrento (or Salerno) 4 nights
Rome 3 nights
Or - same as you have but with the 1 night in Siena
Rome 3 nights
Siena 1 night
Florence 3 nights
Venice 3 nights
Sorrento or Salerno 3 nights
Rome 1 night
PS: The AC is beautiful but getting to/from is chaotic, inefficient and frustrating. It is no more gorgeous than the Oregon coast (and that is from a California girl).