I also like the suggestion of doing Pisa & Lucca together.
Looks like you have 5 days (day light days) in Florence (given you depart by train in the evening) and only 1 in Venice. I would shift 1 day over to Venice. Ultimately, it depends on how many day trips you will make from Florence, and how much time you need for the city of Florence. I do think Venice deserves 2 full days.