We are originally were doing Florence-CT by train. Return to Florence, pick up car and then drive onwards to Tuscany. Now thinking about renting car when leaving Florence for CT. Are the roads OK and are the tolls easy to figure out? Yes, we will have GPS (and a wife with fab. Map LOL).
Have you figured out what you'll do with the car once you get to CT? Have a look at this other current thread:. https://community.ricksteves.com/travel-forum/italy/cinque-terre-with-a-rental-car
Renting in La Spezia when you leave the Cinque Terre is a nice idea. The train from there to say Siena is a drag (We did it. 4 trains.) so driving would be much better since you need a car anyway.