We'll be picking up a rental car at Milan Linate and driving to Volterra. Google Maps shows three possible routes:

1) E35 to near Parma; SS62 to near La Spezia; E80 to near Pisa; SR 206 and 349 to Volterra.

2) E35 via Bologna and Florence (bypasses both cities); SR68 into Volterra.

3) A7 to near Genoa; SS1/E80 to near Pisa; SR 439 to Volterra

All of the routes go through hills or mountains. We have driven mountain routes in the USA and Canada and wonder if there would be any unusual challenges driving any of these routes. Given that they all take 4 - 4.5 hours, which would be most scenic? We will have a whole day to drive from Milan to Volterra and would appreciate suggestions for places we might visit quickly along the way and perhaps stop for lunch. We have visited Florence, Lucca, Pisa, and Siena on a previous trip.

Thank in advance for sharing your expertise!

I would prefer choice 1. There are some nice, little visited towns between Parma and Sarzana worth exploring. Sarzana and Carrara are also worth a visit.

Are you flying into Linate? I have recently read that it is closing for a few months but I'm not sure where I read it???

I have driven them all. I generally don’t find the larger highways in this part of Italy to be particularly scenic. My goal would be to get to beautiful Volterra, one of my favorite Italian towns. I would pick the first option; I believe it's the quickest. Perhaps lunch in Parma?

And, yes Milano Linate is closing late summer into fall for renovations:

https://www.businesstraveller.com/bu...r-summer-2019/

I opt for Parma (visit!) - Carrara - Massa too. Take the old Cisa Pass road if you like a scenic drive over the mountains (some hairpins). If not, take the much faster motorway.
You may also travel via Salsomaggiore Terme if you like to visit a traditional Italian spa (itinerary Fidenza - Salsomaggiore - Fornovo di Taro). Spa of Salsomaggiore