If you rent a van you will regret it deeply.
If I were you, upon arrival to Venice (not sure if you fly in, arrive by train, arrive by ferry, or arrive by cruise ship), since you are not interested in seeing Venice, you should immediately go to Florence by train. There are baggage service porters at the station that can help with luggage. There is plenty of room for luggage on the trains. Once in Florence you can take taxis to the hotel.
The DAY BEFORE YOUR FLIGHT, you should go back to Venice (by train) and check into a hotel for the night, preferably near the station/Piazzale Roma. You can visit a few hours on that day. The morning of the flight you go to the airport (you should be there about 2.5 hours prior to departure is it’s a US bound flight). If you stay near Piazzale Roma it’s only 20 min taxi ride to the airport.
To drive from Florence to Venice on the day of departure is reckless behavior. It’s a 3.5 hour drive under optimal circumstances, but I’ve spent also countless hours driving from Florence to Venice under less favorable circumstances (e.g. freeway accidents or car trouble). Shyt happens, unfortunately, and I wouldn’t want it to happen just before a transatlantic flight. I’d rather get up at 6:30am in a Venice hotel, than wake up at 2:30am in a Florence hotel to drive to an airport over 200 miles away.
The cost is also not more if you do the above by train. A 9 pax van will cost you $150 a day to rent with mandatory insurance. Add fuel, freeway tolls, parking fees in Florence, and your van adventure will cost you at least $800-$900.
The full base fare Venice to Florence is 57€ one way per person. So even by paying full price the 6 of you will stay under $800. If you buy tickets in advance you can save up to 60%.