16 days critique of my itinerary in Italy

For what you are proposing, a car would be a handicap. You forget to allocation time for find parking space for the car. You are doing some back tracking. I would try for more of a straight line. Fly into Venice and come home from Rome -- save time and money. Venice deserves more than a day. I would drop Umbria and expand time in Florence (Tuscany) and Venice. I am not a fan of day trips to Pompeii since it is a lot of time traveling in relation to time on site. If going to Pompeii you need to work in time for the museum in Naples where all the good stuff from Pompeii is on display.

You have a great trip planned; here are some suggestions, based on our last 2 trips to Italy--they're just our own perspectives, but you may want to consider them.

Rome - I would give more time there; there is so much to see in Rome, and although 3 full days will be great, you might consider taking a day from your time in Tuscany and putting it into Rome.

We were in Tuscany this May; we did enjoy it, but in my opinion, there's just not enough there to warrant 4 full days after the landing day in Siena.

If it were me, and I were going to the cities in your itinerary, I might consider something like this:

Day 1: Land in Rome - Explore the City
Day 2-5: Rome
Day 5: Daytrip to Pompeii - you should visit the Amalfi coast if possible while you're there
Day 5: Rome
Day 6: Train to Venice
Day 7-9: Venice
Day 9: Land in Tuscany (Homebase in Siena)
Day 9-11: Tuscany
Explore the towns

As you can see, I added a day in Rome, and also a day in Venice--but that's just me... :)

We haven't been to Umbria so I can't really offer an opinion on that, but here's a trip I've sketched out that includes some of what you're wanting to see:

Venice - 4 nights (so 4 full days)
Rome - 3 nights (3 full days--we've been before so might only stay there 3 nights)
Naples - 4 nights - time to see the Amalfi coast, revisit Pompeii, get out to the Isle of Capri

Our plan is to use the train between cities, then after Naples head to Greece for a week there.

Rome to Pompeii and back on a self-trip you can arrive at Roma Termini at 9AM and return at 6:15, checking current schedules for any minor changes: https://www.trenitalia.com/en.html

Licensed guides at the ruins conduct small group tours.

Roma Termini to Napoli Centrale on Trenitalia 9:15-10:23, then downstairs to Piazza Garibaldi and tickets [window or news stand] for the next Circumvesuviana train to Pompeii Scavi, arriving at 11:47 or 12:11. Then back to Garibaldi at 4:12 to 4:44, train to Termini at 5:25-6:35. Earlier trains to Napoli at 7:00 and 8:00 [all 1:10]; later returning at 6:09-7:20 [From Pompeii 5:00-5:38].

Or you can take trains on Italo: https://www.italotreno.it/en 

Tuscany with a car do not stay in Siena; pick a town in the Val d'Orcia:

https://www.wanderingitaly.com/maps/valdorcia.html

https://tinyurl.com/5eum2na7

Trip report: https://tinyurl.com/4fsms3wk

Without a car, I suspect Florence would be a better base for Tuscan day trips than Siena. You might also consider taking a one-day, small-group tour that hits several small Tuscan towns. I haven't done that, but Tours by Roberto gets great reviews here. One thing that's challenging without your own car is putting together a day with quick visits to multiple little towns--places that don't necessarily need more than 2 hours but have so little bus service that you may have a choice between skipping lunch entirely or staying for 4 hours.

Tuscany and Umbria are both beautiful, but for me, 7 days visiting small towns in those areas would get a bit repetitive, especially if the focus is primarily on the little places, rather than Perugia, Orvieto, etc.

My first thought is why are you doing round trip to Rome? Venice now has great (probably better) flight options than Rome. Fly into Rome and home from Venice or the opposite direction, depending on where you plan to go after Rome.

After Rome train to Orvieto. It's a nice town for two nights (one full day) or arriving early, spend the day and one night, and leave late the second day. You can bus to Bagnoregio to see Civita. Personally, I'd skip the dead town, enjoy Orvieto, and see more hill towns in Tuscany. If Assisi is high on your list, definitely visit there instead. I consider Assisi a must for St. Francis admirers, not a must for everyone else (totally personal preference). Either way, you probably don't need as many nights in Umbria.

Train to Florence, visit Tuscany area from there. I have trouble with this suggestion. Florence has absolutely wonderful art, maybe more than all the rest of Northern Italy put together. That said, it's my least favorite place to stay and eat in Northern Italy (some people will take exception with that, personal preference). Rather than pay extra to stay in Florence, so you can visit other places, I'd prefer a rental car and picking two towns to base for two nights each. You can drive around during your full day to stroll one or two towns, then enjoy evenings/dinner in your base towns, or you can just relax in your base town, see the sights without rushing around. If you prefer the latter, you could stick with public transportation (busses run everywhere but not always conveniently).

Finally, train to Venice and enjoy. Two nights gives you one full day. I'd prefer three nights to give you two full days to explore. Then take Alilaguna water boats to Marco Polo for your flight home. There are also train and bus options to Marco Polo.

I think 3 nights is a reasonable minimum for Venice. Truly, a lot of folks seem to go there for just one day (2 nights if they're lucky) and end up disliking the city because they've spent all their time going to the same places as the daytrippers, just part of the mob. Venice's charm is in the back streets and along the tiny canals. Give yourself time to experience it. Enough time in Venice is really more important than seeing a bunch of small Tuscan and Umbrian hill towns. (Don't get me wrong, I like Orvieto and Assisi a lot.)

Fly into Venice. 3N.

Train & Bus to Siena. 4N.
Rent car. 7 days.
Personally, I’d look at some of the smaller hill towns, Siena can be a pain to get in bc and out of.

Drive,Siena to Assisi much faster than backtracking for the train. Umbria 4N.

Train to Rome. 5N. Including a day trip to Pompeii.

Fly out of Rome.