Best train/bus options?

Venice to Rome (it looks like a flight would be fastest and same price as the train?)

I'd suggest that a flight is unlikely to be faster overall.

Train - exactly 4 hours, middle of Venice to middle of Rome.

Fly - about an hour overall to Marco Polo airport + 90 minute check in + 1 hour-ish flight + 1 hour overall to the same place the train takes you (train from the airport to Roam Termini station) = 4:30, or 30 minutes slower than the train IF everything is to clockwork including the flight.

Train - walk on, walk off, relax in the middle. Big windows to watch the scenery (at 300 kph) zip past, toilets in every carriage instead of the couple at the back of the plane. No luggage hassles.

Fly - all those separate elements, the stress, the line for security and all the fun that involves, a tiny window if you are lucky.

Trains purchased ahead can be very cheap,

Transportation options between locations are well served by Google Maps. Input your start and end points, and estimate the departure time of day. Since March bookings are still too far out, select the day of the week a week or 2 from now. You will see listings for car and the various public transportation options. Of course, go to the actual provider for bookings. The Man in Seat 61 website is excellent for learning about the train systems.

Rome to Sorrento: train to Naples, then local Circumvesuviana train or hired car and driver.

Sorrento to San Michele. The bus (buses) are very time consuming. A hired car and driver , while taking the same road, will likely get you there in half the time.

San Michele to Orvieto will be a long day with multiple changes of buses and trains. You could cut the time by a couple of hours with a hired car and driver to the train station in Naples.

Orvieto to Florence: trains with required connections

Florence to Riomaggiore: trains with at least one connection

Riomaggiore to Venice:trains with multiple connections

I agree with Nigel. Trains in Italy are inexpensive and much faster as they get you right to the center. Trenitalia website on a laptop is a little easier to use. Honestly, in the past I’ve just purchased the inexpensive train tickets and it worked great. If you don’t want stops or slow train, opt for the fast ones. And man, THEY. ARE. FAST!! Also, the train is just fun! You get to see the countryside. Enjoy your trip!

And keep in mind that the cost of flying is not only the cost of the flight, you need to add the cost of travel to/from the airports,

….not to mention the carbon footprint cost that comes with flying when trains and buses can get the job done.