My first recommendation would be to head directly from Venice to Rome on the 16th and have 3 full days in Rome.
Keep in mind that Rome is rich in history and culture - so is Florence of course - but Florence does not have the same 5,000 year history that Rome has.
In an ideal world, you would see Florence in 3 days but in Rome you'd need 2 weeks.
(which most people don't have, of course, but just to give you an idea of the scope of what there is to see.)

Otherwise, I'd recommend 2 nights in Florence and 2 nights in Rome, which means one full day in each city.

Pisa - honestly, I would leave it for a future trip. It is set out of the way, and I'm not sure your time is best spent there.

Always keep in mind how many full days you have in each place. Travel days are tiring and can be stressful. I always try to reduce the number of days changing hotels, because packing/unpacking and checking in/out of hotels takes up a lot of time and effort.

Happy planning!