Help finishing off 2-week itenerary: Florence/Tuscany + Rome + ?

Both Florence and Rome require a minimum of four nights if taking a day trip from Florence to Siena; you don’t have time to explore Tuscany so no need to sleep in Siena too.
If you plan on visiting Vatican City don’t plan on day tripping from Rome. If the Vatican is of no interest to you then a day trip to Orvieto by direct train is feasible from the Roma Termini station (1h 15m).

Venice could easily absorb another night; that leaves a 3 -4 split for Florence -Rome.

Trying to cram in some additional locale would be ill advised

What time do you arrive in Venice late morning? If you’re able to sight-see on arrival day, try to see as many highlights by following the crowd. The perfect way to end the day is to take a vaporetto (water bus) from one end of the lagoon i.e., the train station to San Marco Square or even further at twilight. This way you’ll see the venetian palaces that line the Grand Canal in its glory but not the decay. Seeing the illuminated interiors will take your imagination back in time. The next day get lost on the lagoon. When you’re ready to get back on the beaten path find the crowd.
Another idea is when taking the train from Florence is to get off in Orvieto (2h 15m) and store your bags at the train station and explore Orvieto before continuing to Rome. You’ll need to get an early start and be organized so a good guide book would be needed.

Good tips Mary, thank you!
We land in Venice at 9:30am so we'll have that day to spend there, though I'm sure the jet lag will be heavy.

I like the idea of getting off the train and stashing our bags for a quick stop. Will we need separate tickets for each leg of that, or is the one ticket good all day?

4 nights in Rome and Florence is 3 full days in each. The train from Florence takes 1.5 hours to Rome. I'd leave Florence early to have most of the day in Rome. Of course that leaves you less time in Florence, since you'll lose the morning transferring from Bolzano. 3 full days in Florence gives you time for the highlights and a day trip to Siena (good choice). Although if you want to "see" Tuscany, take an all-day wine tour from Florence. Siena is a lovely town to visit but you won't see much on the bus and the only good Tuscan view is from the tower.

Going to Orvieto on the way to Rome isn't time-efficient. The train from Florence takes at least 2 hours and another 1.5 hours to Rome. Also, I'm not sure there's luggage storage at the Orvieto station.

Less is more. You lose too much time changing locations and you have less time to enjoy each place your at. Italy is not a once-and-done destination.

Going to Orvieto on the way to Rome isn't time-efficient. The train from Florence takes at least 2 hours and another 1.5 hours to Rome. Also, I'm not sure there's luggage storage at the Orvieto station.

Sam, I took another look at trains and assumed the train stopped in Orvieto but there are direct fast trains that don’t taking 1h 30m. I suggest skipping Oriveto as a stop along the way. However, according to this website there is luggage storage in Orvieto: https://www.orvietoviva.com/en/orvieto-train-station/.

Should we just stay in Florence for 4 nights and Rome for 4 nights?

Yes.
Well THAT was easy! 😄

I just spent a week in Venice in May, so I don’t think two nights is enough there as it will only give you one whole day. Personally, I would do thee night in Florence and Venice, and four nights in Rome.

Just wanted to update everyone since I got so much help here, thought this could be helpful to people looking in the future.

We ended up doing this:

Day 1: arrived in venice 9am, got to the hotel around 11, napped and explored.
Day 2: venice walk around
Day 3: drive from Venice to Cortina (slept in cortina)
Day 4: hike Tre Cime (slept in cortina)
Day 5: Drive to Ortesi, hiked Cinque Torri along the way. Amazing drive, so scenic. Ended up being able to go up alp de susi for sunset apertif that night for a special event
Day 6: Went up Seceda lift
Day 7: drop the car in Bolzano and took the train to Florence.
Day 8,9,10: stayed in florence. Did a half day wine tour in Chianti, and an almost full day bike tour from Montepulciano to Sienna
Day: 11 train to Rome, went to the Forum, nice dinner
Day 12, 13, 14: Explored Rome
Day 15: flew home

Highly recommend this itinerary to anyone interested, it was amazing. If you have extra time, I could have stayed an extra night in Ortesi, it was really nice there, a much nicer place to stay than Cortina, to my surprise. Hotel Stua Catores was a highlight. Definitely glad we had the rental car for the Dolomites leg.
Nothing felt rushed. I'm honestly glad we only did 2 nights in Venice. Beautiful city and fun to look around, but definitely felt like disney land. I'm glad we went there but 2 nights was plenty.

Planning took a lot of energy but it paid off! The trains were easy, BUT highly recommend making reservations in advance, didn't realize they would be mostly full.