Venice itineraries fail when they try to do St Mark’s, a gondola, and Burano before a 4pm train. Stay the night.
2 nights: canal and cicchetti
Day one: Line 1, St Mark’s cluster, Dorsoduro dinner. Day two: wander, Rialto morning, Accademia Bridge sunset. Arrive from Florence (about 2 hours).
3 nights: add the islands
Keep the two-night core, then Murano and Burano. Torcello if you want the empty lagoon.
On the 10-day Italy circuit
Nights 6–7 after Florence, then south to Naples and the Amalfi Coast. Do not day-trip Venice from Florence.