Coutryside road trip Italy

we would rather focus 80% of our time on the less known and not so crowded places.
You don't say what time of year you'll be traveling.
If you want a more authentic experience, avoid going during high season. The shoulder seasons of Spring and Fall are nice. We actually went in December and had a great time. We ate frequently at a restaurant frequented by townsfolks and the owner would come and visit with us during dinner.

The CT everyone wants to visit for the most part doesn't exist any more, especially in the summer. Between San Gimignano and Volterra, I prefer Volterra. Consider staying in an agriturismo instead of a village.

I still think you're moving around a lot. I'd do Camogli or CT, not both. If you are flying out of Rome, wait and go there last. You don't want to have a car when in Rome. I've never been south of Rome, so I can't comment on that, but even with 18 days, I think you are trying to cover too much territory.

2 nights = 1 full day
So, 1 day in Florence then off to the coast. I really like Portovenere and you can catch a boat to the 5 Terre towns. you could see Lucca and Pisa if you don't go there from San Gimignano.

San Gimignano. Rent a villa for 5 days near there. See Volterra, Siena, Certaldo, San Gimignano, Monteriggioni, and even Pisa and Lucca if you have a mind to drive that far.

Yes, drive through Umbria, see Orvieto, Montefalco, Spello, Spolleto, or others, your choice. dump the car and catch a train to Salerno, or drive.

I don't know that driving to Ravello is such a good idea, and certainly not Montepertuso (great restaurant there , La Tagliata). Think about going from Salerno by some other means, driving along the Amalfi Coast is not something that is enjoyable for most.

Then, as suggested above, return to Rome is that is your departure point. See it last and see it for at least 2 full days (3 nights).

Now having said all that, you want to "focust 80% of our time on the less known and not so crowded places." Well, I hate to tell you this but all the places you mention do NOT meet that criterion. All the places you mention are usually crowded and are very well known, but worth the visit for sure.

What month?
Are flights booked? Is flying open-jaw an option? You will need to be in Rome or nearby for the last night, so try to consolidate your Rome time at the end.
I would omit Cinque Terre, as it is the most opposite of not crowded. Outside of July-August, every other coastal place I have visited has been lovely and uncrowded. Three days in Camogli would not be too much. I stayed 4-5 and never ran out of things to do.
Driving on the Amalfi coast in season could put you in a long traffic jam. I'd skip it for only one night and stop somewhere like Paestum if you need to break up the trip. Pisciotta is wonderful.