We are considering 3-4 nights in Florence, 2-3 nights in Milan and 1-2 nights in Cinque Terre...
We are considering a day trip to Siena and a day trip to Lucca?
From where will you take your day trips? Milan is too far away and you don't have near enough time for Florence already. 3-4 nights means 2-3 days.
If you want to see Florence and Tuscany, then spend 4 nights in Florence, rent a car and spend the rest of your time in an agriturismo near Siena. Leave out CT altogether. If you must, you could day trip to Lucca from there. It's a ~2 hour drive. However, I recommend exploring towns nearby, such as Radda, Castellina, Greve as one day exploring. Then San Gimignano and Volterra as another day and Montalcino and Montepulciano together. Spend an entire day in Siena.
What time of year will you be going?