Driving round trip Pienza to Porto San Stefano

The fastest way is to head to Grosetto than taking the E80.

The most scenic would be thru Bango Vignoni/Radicifani/Sorano.

I'd try to work in a stop at Pitigliano (the Sorano route).

Holy cojoni, a day trip from Pienza to Giglio is already plenty ambitious for a day trip to even consider any stops en route.
My suggestion is to get to the ferry the fastest possible way, which is basically going through Montalcino and San Quirico and eventually reaching Paganico to get on the E78 (Grosseto-Fano) and then the E80 from Grosseto to the Albinia exit. If you make it in two hours you qualify as an honorary Italian driver.
Once in PSS, there is a parking lot next to the ristorante La Bussola (recommended for fish by me), where you park while you go to the island. The ferry dock is across the street. The ferry ride is just over one hour. Once at Giglio Porto, and maybe have lunch there and look at the shops, I presume you want to walk or take those taxi vans to the Spiaggia della Cannella (nice beach). The walk is quite a walk but doable, a taxi is a 5 min ride. Once you spend some time at the gorgeous beach della Cannella, and take the ferry back, it’s likely past 6pm (don’t miss the last ferry which I think departs at about 6 or so.)

After a full day in the sun I’m not sure you want to stop anywhere else, you still have 2+ hours to drive back to Pienza. I’d be more concerned to stay awake at the wheel on the way back.

Thanks for the suggestions!! We’ll be staying a night on the island so we’ll have a bit of time to enjoy the journey.

I was assuming you would have at least one night on the island. The driving is windy on the promontory and inland, but the main highway north and south is quick.