What a great trip- lucky you to have so many nights!
It feels like a few spots need a bit of optimizing.
The Florence-Venice-Florence-Siena bit is awkward.
Time split on Capri makes no sense to me.
Will you be using the car while in Siena for driving around Tuscany?
I see no need to drive all the way to Sorrento. Stopping for lunch will probably be a bit stressful- parking, ZTLs, luggage in your car, etc.
I think it would be smoother to either go to Venice from CT then back to Florence for your Tuscany time
or
Go to Florence/Siena from CT then to Venice then train all the way to Salerno.
We took train from Salerno to Venice (about 6 hours) and actually really enjoyed it. Took a picnic aboard and it was a nice rest day after our 9 nights in Amalfi.
Easier to get from Salerno to Ravello- ferry to Amalfi then bus or taxi.
Ravello is lovely but 3 nights there is possibly too much, it is quite dead in the evening and the transportation to other Amalfi towns is a pain. Might cut 1 night from Ravello and give to Sorrento or Salerno, along with those 2 nights in Anacapri.
(we did 4 Sorrento, 2 Amalfi, 1 Ravello, 2 Salerno)
You don’t mention Pompeii- are you interested in visiting?
What about Paestum?- really amazing Greek temples well worth a visit. Easy train ride from Salerno.
Assuming you will actually USE the car while in Siena
Options:
CT 3 nights
Pick up car in La Spezia- to Siena 3 nights- drop car as you leave Siena- be sure your lodgings have easy access parking
Train to Florence 3 nights
Train to Venice 3 nights
Train to Salerno- then to Ravello or spend a night or 2 in Salerno to visit Paestum before heading to Ravello
Capri- 2 nights
Sorrento- 3-4 nights- visit Pompeii, Naples?
Train to Rome
or
CT 3 nights
Train to Venice- 3 nights
Train to Florence 3 nights
Pick up car as you leave Florence- Siena 3 nights
then drive as far as Chiusi to drop car and catch train to Salerno (3-4 hrs)
(Or you can just take bus to Siena-rent your car in Siena for just the days you will use it, returning in Siena as well then train to Florence-Salerno)
Car rental rates are high this year so strategic rental of a car for just the days you'll really need/use it is wise- be sure to check days of week and open times- car rental places close mid day and short hours or closed on Sat/Sun
We always rent with AutoEurope.com- a US based consolidator with stellar customer service and usually best rates.