Sardinia

No advice on Sardinia but you don’t want or need a car in Florence
Parking, ZTLs, etc

Take the train

Ditto to the above: take the train. You don't want or need a car in Florence.

I would fly to Sardinia instead of taking the ferry. Many auto rental contracts forbid taking their cars on ferries. And by all means a rental car on Sardinia is a must. It was less expensive to rent and return a car at the Cagliari airport than from a downtown office. I rented from Europcar and they did allow taking the car from Sardinia to Corsica on the ferry. With ten days you might consider three or four days on Corsica. I covered both islands and hope to return again.

Do not drive from Venice to Florence. The train is way faster, and cheaper too.
Also once in Florence the car is useless (just like in Venice) and you’d need to keep it parked in a garage the whole time. Florence city center is closed to non residents’ vehicles.
Fly to Sardinia from either Florence or Pisa. Train to Civitavecchia plus ferry to Sardinia takes too long.
Rental cars are allowed on ferries connecting to Italian islands, but the cost of ferrying the car across to Sardinia (about €150-200 one way) is more than the airfare for 3 or 4 people. Rent a car when you land. You will need one there. You can fly to Cagliari in the south, or Olbia in the northeast from both Pisa or Florence. Pisa also has flights to Alghero in the northwest.
10 nights in Sardinia is good. The island is large and most roads are two lane curvy highways, but in 10 days you can cover a good portion of it.

https://community.ricksteves.com/travel-forum/italy/sardinia-a4b6bf34-12f0-4f41-aa46-cc7c08ab1f7f

Roberto what happened to the ferries departing from Livorno?

Livorno to Olbia (Grimaldi Lines) or Livorno to Golfo Aranci (Sardinia Ferries) should still be there too. But from Livorno it is a longer ferry journey compared to Civitavecchia. it’s a 9 or 10 hours crossing, instead of 6 from CVA, with prices still in the same range to ferry a vehicle.
Flying from Florence (Vueling or Ego Airways) or from Pisa (RyanAir or Volotea or, maybe also ITA) takes only an hour (plus check-in and boarding time) and I’m sure it won’t cost more than €100 one way, even with the most expensive fare. The only issue is that non stop flights to Sardinia are for the most part seasonal and maybe not daily from Pisa or Florence, so one has to choose the proper time of travel. Although flights to Cagliari are probably all year around. I haven’t checked.