26 days of Italy.First timers

Just a reminder. You mentioned "Tuscany 2 nights car". That means one day, not nearly enough time for Tuscany. When are you going? Tell us about why so long in Sicily compared to the other areas.

Aug 19 - Sept 14
I just thought it would be nice to cruise around Sicily in a car and chill abit and take it in?
But hey open to suggestions.
I could do 1 more night in Tuscany. Any suggestions on where to visit or stay while there?

Since you’re flying in and out of Rome sleep in Rome at the end of your stay.
What time do you land in Rome? The reason I ask is because there are direct trains departing from Rome’s Fiumicino airport train station to Naples central station and Salerno. You can arrange a private transfer from the Naples train station or Salerno train station to where you’re sleeping on the Amalfi Coast.
From the Amalfi Coast take a ferry from either Naples or Salerno to Palermo. Make sure you take a day trip to Cefalu from Palermo. When you are ready to leave Palermo take a train to Siracusa that requires a minimum of one transfer.
Fly out of Sicily from Catania to Venice and rent a car at the airport and drive to the Dolomites. From the Dolomites drive to the Cinque Terre and park your car. From the Cinque Terre drive to Volterra and spend the night. From Volterra drive to Siena for two nights. A good day trip from Siena is Pienza’s old town and Montepulciano. Make sure you drive along the Val d’Orcia: https://lovefromtuscany.com/scenic-drives-in-tuscany/. From Siena drop off your car Chiusi and take a direct train from Chiusi to Rome’s Termini station.
What time of year are you taking this trip?

Are we missing something we should really not?

There is nothing 'wrong' about you plan but it is atypical in that Rome has only 2 full days; Venice and Florence are missing.

Given the time of year you are there CT and Amalfi Coast area are going to be the proverbial 'packed like sardines' situation.
Tune your expectations to deal with that reality

Re Sicily, I haven’t been there (yet) but from the experiences of others, 7 days would not be too many if it is an area of interest. Often folks mention a one week stay exploring either eastern or western Sicily as it is quite a large area with a lot to offer.

Mid august to mid Sept
I was thinking going north first might be best as Amalfi coast may be alittle less crowded at start of Sept than mid August?

Fly into Rome at 8 pm from Australia so could be pretty pooped by the time we get there and may need a god sleep before kicking off, so thought Rome first could be a good way to get feet back on the ground

I would not base in Positano
Transportation from there is iffy, ferries can be cancelled due to rough seas because it’s a small dock

Amalfi makes a better base
Or Salerno
Or Sorrento

Depends on what else you want to see/do while in the area

Tuscany needs more time