Wisdom of Trip to Venice

If you're asking if it's doable, yes. Take the earliest train possible from Florence 2hr05min.
Then the next afternoon train to Rome 3hr45m
Edit: you're spend the night in Rome?

Just to confirm, you fly home the day after your return to Rome.

This will give you a bit over 24 hours in Venice.
The big downside is you'll be there at the most crowded times during the day.

Can anyone suggest the best way to accomplish this excursion.

Add more days to your trip. Otherwise, spend the day in Florence to see a few more items not part of the tour, take a late afternoon/evening train to Rome and give yourself a bit more time there. It sounds like you are committed to fly out of Rome on a specific date, so trying to work Venice in the time you have will likely not be truly enjoyable , but more a mad dash to try to 'cram' it in

Save Venice for the next trip ( good reason to have to go again!)

It is doable. You don't have to rush back to Rome. The next day you can take a train straight to FCO.

Trenitalia’s high-speed Alta Velocità (AV) trains connect Venezia Santa Lucia station with Rome’s Fiumicino Aeroporto station. But they only do so directly once per day. There are 29 trains per day leaving Venice bound for Rome’s airport, however there is only one train per day that offers a direct route. the majority will require a train transfer.

The average journey time between Venice and Fiumicino Aeroporto is 5 hours and 32 minutes and the fastest journey time is 4 hours and 15 minutes.

https://www.italiarail.com/pages/routes/venice-to-rome-fiumicino-airport

yes, go to Venice; it is pretty spectacular. Know how to avoid the crowds.

Personally, I wouldn't do it. I'm a big fan of Venice but would want to have enough time there to get away from the mobs, and that definitely means more than one day. Six hours on trains (plus time going back and forth between hotel and train station) for a bit over 24 hours wouldn't be worth it to me.

The tour gives you less than two full days in Rome and two full days in Florence. Since the tour ends in Florence and you fly home from Rome, you have an opportunity to add time in either or both of those cities without extra train time. You'll just need 1-1/2 hours for the Freccia between Florence and Rome. I have a hard time imagining you'll have run out of things to do in those two major tourist destinations after two days each.

Really love Venice, but have to agree 100% with acraven's. The time in Venice will not (in my opinion) be worth all the travel time.

I would do it - if you can get a night AND the next morning. These are the least crowded times in Venice, when it doesn't feel so overrun. I think morning is my favorite time. That way, you can see shopkeepers opening up and sweeping in front of their stores, chatting with each other; locals walking their kids to school., etc.

I wouldn't do it otherwise - you need at least a night and a leisurely morning to get enough out of it.

If you love Venice, come back next year and see more of it, as part of another trip.

I would give it a shot, too. Even though your time there will be all too short, you will get a taste of what you will want to come back for-and schedule a longer visit! It's a magical city.

Personally, I would split the additional time between Rome or Florence and save Venice for another trip when you can do it justice. But a year ago a friend of mine did pretty much what you were describing and loved it. It was her first time to Italy. Just go with your gut and don't fall victim to FOMO.

Add a few more days at least so you can really see Venice and not travel there for just one night.

I would save Venice for another trip. What you contemplate will be more a chore than anything.

Enjoy your extra time in Florence. There is so much you won't cover on your tour. Or Rome.

Enjoy!