questions on travel within italy and itinerary

1. for the sorrento to pompei to rome part, if I leave sorrento as early as I can, what time should i buy tickets for the pompei-naples-rome portion? I know the real answer is "it depends on how interested you are in pompei" but a general guide will be greatly appreciated. Right now, I'm leaning towards catching a 6:30 pm departure. To my understanding, I don't need to prebook Sorrento-Pompei. Please let me know if that's not accurate.

The Sorrento to Pompeii and Pompeii to Naples is a commuter train. No need to pre purchase.

Naples to Rome a pre purchase can save money. You may want to decide what time you want/need to arrive in Rome and work backwards to determine when to leave Naples. That will dictate how long you have at Pompeii.

2- It may be easier to take a bus between Siena and Florence

It’s easy to visit Siena from Florence on your own - you can take the bus or the train. No need to reserve. Bus is better ( faster, lets you off in the middle of town rather than at the bottom of the hill), but the train is ok too.

You are spending only one night in Venice? I guess you've been there before. If not I would plan for 2 or 3 nights there as well.

Your questions:
1. The Sorrento-Pompeii-Naples portion will be aboard a commuter train, similar to commuter trains in NYC, so no need to purchase in advance. Just buy the morning you go.
The portion from Napoli Centrale - Rome will be aboard high speed trains. If you buy in advance you can save money, however if you do you cannot make changes. Discounted tickets are not refundable. Since the portion is not that long, you could also buy that ticket when you get to Naples and pay the full BASE price. It's up to you. The visit at Pompeii should be about 3-4 hour long. You can spend all day, but I think 3-4 hours is good.
2. No. Pisa-Florence is aboard Regional trains. No discount for early purchase. Just buy on the spot (ticket office, ticket kiosk, or even with the App). Also the Spezia-Pisa is likely to be aboard a Regionale (although there are some intercities also). I would stick to a Regionale because they are cheaper and you don't save much time on IC for such short distance. Also, coming from SP it is better to get off at Pisa S. Rossore which is close to the leaning tower. However if you have luggage, the luggage storage is only at the farther Pisa Centrale station (30 min walk to the tower). To visit tower, cathedral, baptistry, a couple of hours would be sufficient.
3. Bus from Florence to Siena is best. 75 min ride. No need to drive or hire a driver. It's a simple trip. The bus station in Florence is across the street from the train station on the Bar Deanna side.
Italians are not used to talk North/South/East West, the West side of the station is called Bar Deanna side, from the name of the homonymous bar. (That bar is still there and has been there since I was a child. If they close it or change the name, half of the population in Florence will be lost and bus schedules will need to be changed, because they all refer to Bar Deanna).

You might want to start by getting a map of Italy. Most travelers start in Rome as the best airport is there. They take an early train to Naples, and catch a connecting train to Sorrento. For all practical purposes, Sorrento is the Amalfi Coast. Then they'd catch Pompeii on the return trip back to Rome.

From Rome, most take the train into Florence. From there, it's easy to go over to Pisa, but The Leaning Tower is not really worth the effort to see to most people. I'd be gong straight to Manarola thru LaSpezia.. Then back to LaSpezia to catch a train to Venice. I don't know if they even have a night train, but I'd avoid any night train in Italy.
And yes the day trip to Siena is usually best done by bus--caught across the street from the Florence train station--to the right of McDonalds Restaurant. The let you off in the center city.

Roberto,
I am only spending one night in Venice. The train from Florence arrives at 9:34 AM and then the train to Munich the next night leaves at 9:48 PM and i booked a hotel that is right near the train station that said it would store my bag pre/post check out. So it's almost two full days though it will definitely be tiring. Maybe i made a bad decision and should have added a day, oh well. I plan on going to the palace and basilica but no museums in Venice (let me know if there’s one or two I should go to) so i thought 2 days would be enough.

As for Pompei/naples/rome, right now I can buy a fast train for about 28 euros that will take 2h30min-2h50min for the whole trip or a slower train for about 16 euros that will take 3h17min. I think I’d rather save the euros and take the slower train. Do you know if all trains from Naples to Rome have a changing price that will get more expensive as the date approaches? Or do the slower Naples/Rome train have a stable price?

Ok, so I won’t buy a Pisa-Florence ticket in advance What about a La Spezia to Pisa one?

For the Siena day trip, would it be possible or advisable to add another city also? A lot of the tours do Siena and another place (I think San Gimignano is a common one) or even three places.

Thanks so much for all the help Roberto!

David,
The original plan was pretty much what you said, arrive in Rome, then down to Naples/Sorrento, then back to Rome, then Florence, then Cinque Torre, then Venice. When I looked at the train schedules though, it seemed like changing it to the way i booked it mad more sense (maybe I miscalculated). Landing in Naples saves me a Rome/Naples train ride, and then later I do go to Pompei on my way to Rome (as you recommended) without backtracking. If I remember the Cinque Torre/Venice leg was the problem as if I remember there were neither fast trains nor direct trains. So it made more sense to do Rome-La Spezia-Pisa-Florence-Venice than Rome-Florence-La Spezia-Venice even though a map makes it seem like the second way is better. The Rome-La Spezia trip is only 3h15m while the fastest La Spezia-Venice route was around 4.5 hours. As I said though, maybe I missed something or didn't notice a better train.

Thanks for the info on Siena, I’ll definitely do it on my own.

in my opinion siena is an all day excursion, any city added might be too much

Yes, bus tours offer what they can sell, and people think it's better to go to two places rather than one, or three places rather than two. I wouldn't recommend a bus tour that combined Siena with another place, but if you just had to see two places that day, a bus tour might be necessary. When you start stringing together multiple smaller places, you sometimes run up against infrequent transportation that causes you a lot of grief. When I go to tiny places (which Siena most definitely is not), I do like to see more than one a day, but I am often faced with the unattractive choice of either skipping lunch (because the bus is about to depart) or spending an extra 3 or 4 hours in a town that I've just about finished seeing because there's a long wait for the next bus.

1. for the day trip to Siena part, is that something I can just do on my own with local trains? Is a tour recommended?

I don't think a tour is needed for Siena, You can take the bus or train. However, this Walks of Italy Day Trip to Siena looks enticing. We've taken several Walks of Italy tours and have been impressed. They are well worth the money.

....Do you know if all trains from Naples to Rome have a changing price that will get more expensive as the date approaches..... You need to understand the pricing structure of Italian trains. Not like US airlines. There are three pricing levels in second class -- those prices never change once set -- Super Econ, Econ, and Base. The first two are obvious discount and come with lots of restrictions. When the train schedule is released the prices for SE, and E are set AS IS the quantity or number of tickets available at that price. The prices and numbers vary by train, route, time of day, etc. Once that price group sells out, that is it. More are not added later. On a couple of rare occasions I have been able to buy an Econ ticket a couple of days prior to travel. But usually the SE and E tickets sell out well before the travel date. The price of the base ticket never changes and is always available unless the whole train sells out -- again very rare. A similar pricing structure exists for first class.

Avoid buying Regionale train tickets in advance on the web. That is actually a disadvantage to you as it reduces your options. A Regionale train will never sell out. If you cannot find a seat you stand till one is available. On all other classes of trains in Italy, seat reservations are required but the Regionale trains are buses on a rail. But they are cheap and convenient.

I don't understand which trains you are looking at for Naples to Rome. I'm looking at a random date of April 13. The fast (frecce) trains take 70-75 minutes and the super economy price available for some of them is 24.90E. The regionale trains cost 12.95E and take less than 3 hours.

When planning that day, take into account the time it will take you from Pompeii to Naples. The Circumvesuviana ("CV") runs about every 30 minutes. The schedule is a little erratic and the trains often are not on time. It can take you 10-15 minutes to go from the CV platform up to the main station and the train platform. You'll need extra time if you plan to buy your ticket at the station. Some machines only take credit cards and some US cards may not work. The regional trains to Rome appear to be once an hour. If you are set on taking a regional train, it may be a good idea to buy it the first time you're in the station, before going to the CV to Sorrento.

According to this link you’ll need to change buses: https://ask.discovertuscany.com/threads/how-do-we-travel-the-chiantigiana-road-by-bus.3026/.
Rome2rio shows that you can take a bus from the Fratelli Rosselli bus station in Florence to Tondo Lucarell (1h 15m) and transfer to Radda in Chianti (30-minutes) along the SR 222 Chiantigiana Road. You’ll then need to connect again to get to Siena (1h)
The direct bus from Florence to Siena takes (1h 15m) but you won’t have the views.