Our Rick Steve's Heart of Italy tour ends in Florence and we planned on returning to Rome for remainder of stay in Italy but now wish we had booked a night in Venice? We are thinking of taking a train to Venice on Monday afternoon spending Monday night and Tuesday in Venice and taking train to Rome Tuesday night? Logistics/planning suggestions welcome!
That works. You would have about 3,4 hours of train travel each way but certainly workable. There are decent hotels (Sonya, Aberdeen) near Termini. That will make it convenient to get to the airport.
Almost gonna say spend another night in Venice. You started with Rome with almost 2 full days. Give Venice at least 36 hours.
Consider changing your flight and leave from Venice, stay as long as you want.