Driving to Venice from Tuscany is a nice drive, especially if you take time to stop and have lunch (or just visit) at an inviting town on the way. We've done the drive a few times now, most recently last July. Yes, there are speed cameras on the autostrada. But, it's no worse than speed traps here in the US and, in our experience, there are signs alerting you to the fact that there are cameras coming up. Also, once you take the time to notice one camera, you will see the others as you drive.
It is super easy to drop your car off at the Venice Airport then take a bus, vaperetto or train onto the island of Venice. It is much easier to return your car at the Venice Airport then returning it to a rental agency in Florence proper.
You should consider the cost of the Autostrada. It is a toll road. The take only credit cards. Here is a great website where you can calculate how much it would cost in gas and toll charges to travel from one destination to another. https://www.viamichelin.com/web/Routes
Have a great trip!