It’s a decent plan but if you’ve never been to Rome it needs more time.
Half a day to get there from Sorrento only leaves 2 full days for lots to see
List what you want to do in Venice and in Florence, you might be able to take nights from those locations.
Oops I thought you had 4 nights Venice but only 3 so no I wouldn’t take from Venice
Starting in Venice is usually better/easier than Rome
I wouldn’t change the direction
I’d do Florence then Siena
Get car as you leave Florence ( you’ll have more options for rentals in Florence)
After Siena you could drive to Chiusi or as far as Rome to drop car
Or drop in Siena but you’ll have to backtrack to Florence for direct train south
Be sure you do your homework about rentals and driving in Italy
You are trying to cover a lot of ground in just 16 nights.
As much as we love Sorrento- that is what I would cut. It will take you a good part if the day just to get there
We spent 5 nights in Rome on our first visit (that was a 17 night trip) heading back for our 4th visit to Rome next month. The major sights all take good part of a day ( Colosseum, Vatican)
When we visited Amalfi/Sorrento we spent 9 nights in that area
ETA
There are direct trains from Florence to Naples. Then you have several options to get to Sorrento
Shouldn’t have to change in Rome
( lots of posts here about getting to Sorrento, do a few searches and you’ll find)
www.Rome2rio.com is a neat site that will show you your transpo options and give you an idea of transit time
But always confirm with the actual train site
https://www.trenitalia.com/content/tcom/en.html