As I understand your post- you have 3 nights in Florence then 4 nights in Venice? Is that correct?
I would give the 4 night stay to my arrival city- 1st day is jet lag- and the 3 night stay to departure city.
3 nights is just 2.5 days to sightsee.
Florence has a lot of Monday closures so be aware of that.
Have you made a “to see” list for each city- which has the most sights on it?
If possible, change your flight to fly IN to Venice, so much easier.
If not give some serious thought to staying night before departure at an airport hotel. You can still spend the day in Venice- your hotel should hold your luggage after check out time.
Travel to airport hotel just to check in and go to bed. Up at 4:15am? to go right to airport.
Otherwise, I think first bus leaves P Roma at 4:20? That’s probably OK as your flight leaving Venice is not your Schengen exit. Check in desks at VCE probably won’t even open til closer to 5 am.
A land taxi from P Roma is a better choice.
We’ve taken several Eating Europe food tours- not in Florence or Venice- but I would expect their tours there are as good as all the others.
www.eatingeurope.com
Try EATWELL10 for a discount or email them for a RS discount
If you start in Venice see if Alessandro is still doing his cicchetti tours- super fun.
https://www.schezzini.it/portfolio/bartour/
I’m not a big fan of food in Venice so would choose to do a food tour in Florence.
The Firenze Card can be a great savings- you have to do the math. We got our money’s worth out of it back in 2014
You probably won’t see terrible lines in March but will still want timed entries to Uffizi and The David.
Super helpful post here by Aussie- all about transportation in Venice
https://community.ricksteves.com/travel-forum/italy/venice-transportation-to-hotel-ala-from-sanmarco-airport
Getting to Venice from Marco Polo Airport. [Make sure you have seats on the right/starboard side of the aircraft, for a view of Venice as you fly into Marco Polo.]
A paper map of Venice is the best three euro you will ever spend. Sure, people say that getting lost in Venice is fun. Getting lost, hauling your luggage and not being able to find your hotel is nobody’s idea of fun. Buy that map at the Tabac or bookshop at Marco Polo.
Do take a look at your hotel website – there may be info there. Otherwise,
Water taxi will get you close, maybe to, the door of your hotel. Can carry about ten people, will cost about €140 for the boat. No need to book, taxis are waiting at the Marco Polo dock.
Shared water taxi. Minimum of two people. Book in advance, wait at the desk in the terminal for the others sharing the taxi. Costs about €30 per person. Google “shared water taxi Venice”, use the venicelink dot com web site. Will get you to a vaporetto stop near your hotel.
Alilaguna ferry, as per below.
Bus to Piazale Roma, then walk to your hotel, or take vaporetto to the nearest stop. Buses leave from the outside the terminal, tickets at the machine.
Water transport in Venice.
1. Gondolas. Only for pleasure, not for transport. 90 euro for half an hour for the boat, more for evenings.
2. Traghetto – big gondolas, two oarsmen, about ten passengers, for crossing the Grand Canal. Found at Rialto, San Toma and a couple of other places. Cost 2 euro per trip, can save a heap of walking.
3. Water taxis, which are like a floating limo, costs to match.
4. Vaporettos - ACTV. Are like a floating bus, operate on a bunch of routes. Get the app Chebateo for routes and schedules. Vaps do not service the airport. Vaps are not water taxis.
5. Alilaguna ferries, service the airport, run from Marco Polo via a bunch of stops to the main part of Venice, Murano and Lido. They stop at some, but by no means all, vaporetto stops.