rent car for entire 9 day trip or combo of train/car?

I would rent a car in La Spezia (next to Cinque Terre) after your Vernazza stay and return it at the airport the day of departure.

I recommend 2 compact cars for 6 people (3 people per car, or one car full of 4 people and one car with 2 people and lots of luggage). I think 3 cars (2 people each car) would be overkill unless your party consists of 3 separate couples who don't like one another too much.

Since you are spending 3 nights in Tuscany near Pienza, you may not need an extra one in Volterra. You can visit Volterra on a day trip from Pienza, or on your way from La Spezia to Pienza.
I know that after spending one night on Lake Maggiore you will wish you had a longer vacation so that you could spend at least 3 nights there as well.

Thanks, Roberto! Great info. Have never been to Lake Maggiore. Just wanted to stay in a nice spot close to Malpensa airport. Originally was thinking of Varenna, Lake Como, as I've been there long ago and loved it, but it's farther from Malpensa. Where in Lake Maggiore would you suggest we stay? Or if we reschedule or trip, would you suggest 3 days in Tuscany and 2 at Lake Maggiore or Lake Como?

Stresa is where most travelers stay on Lake Maggiore, nice town, access to the islands. However, if easy access to MXP is the main concern,look at Ranco in the SE area of the lake. Nice town, nearer to MXP, lovely hotel, Il Sole di Ranco.

I like to stay in Stresa because it’s quaint, beautiful and very close to the Borromean Islands, the star attraction of the lake. Generally takes me about 50 minutes from Stresa to Malpensa Airport and I’ve never seen any traffic. Since from my home to SFO takes me 30-35 min with no traffic (but at least 50-55 with traffic) I figured that is reasonably close, unless your flight is super early. If you want to stay closer there is Arona, also quaint, on the southern tip of the lake. From there you save 15 minutes drive. I’ve never been to Ranco, which is a little to the north of Arona, on the opposite side of the lake.

My suggestion is to take a longer vacation. Then how to allocate days is no longer a problem.
Nine days is too short for what you plan to see. You are covering a lot of real estate.
But if you can give at least two nights to Lake Maggiore it would be good. It’s a long boring and tiring drive from Tuscany to there, so you won’t be able to see anything on the lake if you spend only one night.

I wish we could stay longer! All of us are leaving kids behind with family. 😊 thanks so much for all your good advice!