As stated, flights are already booked and I'm guessing may not be changeable without a price, although I'd certainly see about that possibility? You'd save some time if you could fly into Venice and out of Rome or the other way around. According to a previous post, it sounds like you live in the United States? Flights from Venice to the U.S. often leave early enough in the morning to cause some transportation challenges so flying out of Rome is usually the recommendation.
If you cannot change your flights, you might consider going directly to Florence on arrival day versus breaking up your stays in Rome? It's not all that far away and would eliminate a travel day later, thus saving time/money. According to your original post, you have the nights of 2/25 - 3/8 in Italy, correct? That's 12 nights and 11 full sightseeing days to work with, and your itinerary could look like this:
2/25: arrival at Fiumicino (Rome), train to Florence
2/26: Florence
2/27: Florence (day trip)
2/28: Florence
3/1: Florence (day trip)
3/2: Train to Venice
3/3: Venice
3/4: Venice
3/5: Train to Rome
3/6: Rome
3/7: Rome
3/8: Rome
3/9: Fly home from Fiumicino
The above gives you 5 nights/4 full days in Florence, 3 nights/2 full days in Venice, and 4 nights/3 full days in Rome, and only 2 hotel moves versus your proposed 3. You could shuffle a day from Florence and add it to Rome but it looked like you had some days trips in the plan from that one so I kept it at 5 nights. You would have to change your current hotel reservation for Florence.
We welcome any feedback, as we are not intimately familiar with what
there is to do in/around each city.
This is where you'll benefit from some quality time with a few guidebooks: go see what's available at your public library and bookshops. We all have different interests so you should choose what to see/do based on yours versus someone else's! The top 10 tourist attractions Rome, Venice and Florence head up every list published for the "Holy Trinity" but you might decide that not all of those appeal and so opt for a few different choices. You'd benefit from advance tickets/reservations to skip long ticket lines at a few very popular, very busy attractions, although they'll be a bit less so during the winter.
What were you thinking for your day trips from Florence? I'd probably do no more than 2 so that there's time to explore Florence itself.