My suggestions would the same as Roberto mentioned. Use trains for the majority of trips, unless you're going to be exploring small towns in Tuscany that aren't well served by public transit.
Could you elaborate on your comments about the trains being "precarious"? I find the trains to usually be the quickest, most efficient and most relaxing way to get around, especially when using the high speed trains between major centres. They travel at up to 300 km/h which is much faster than by car, and no worries about ZTL tickets, parking, fuel costs, speed cameras, etc. A car in the Cinque Terre (especially Vernazza) is about as useful as a boat anchor, and the car will sit in a garage gathering dust and incurring rental and parking charges during your visit there.
It's also important to note for driving in Italy, each driver listed on the rental form must have the compulsory International Driver's Permit, which is used in conjunction with your home D.L. These are valid for one year, and easily obtained at any CAA/AAA office (two Passport-sized photos required, which may be provided by the issuing office). You may find this recent thread interesting - https://community.ricksteves.com/travel-forum/italy/drive-without-idp-at-your-own-risk
You may also want to have a look at some of the other posts here concerning the dreaded Zona Traffico Limitato areas that are becoming increasingly prevalent in many Italian towns & cities especially Florence, which is almost saturated with automated ZTL cameras. EACH PASS through one of the automated Cameras will result in a €100+ ticket, which you won't know about until several months after you return home! There's also the issue of parking tickets, high fuel costs, tolls and automated speed cameras including the devious Traffic Tutor system which measures not only instantaneous speed but also average between two points. Violate either or both parameter and expensive tickets will follow.
If you decide to travel by train or other public transit, there are also some potentially expensive "caveats" to be aware of. For example, locally purchased tickets for Regionale trains must be validated prior to boarding on the day of travel, or you'll risk hefty fines which will be collected on the spot! Validating also applies to Buses, Metro, etc. Tickets for the fast trains have compulsory seat reservations, and can only be used on the one train listed on the ticket. If you board the wrong train by mistake, again hefty fines!
Good luck with your planning!