Mainly feel that Adelaide back to Melbourne to 7-8 hours, a little tired, and how long does it take to drive from the Great Ocean Road to Adelaide ?

You can navigate to see how many miles your route is, and set a rest point in the middle

On the first day, visit the main attractions of the Great Ocean Road. On the second day, head to Robe. On the third day, go to Victor Harbor. On the fourth day, go to Kangaroo Island. It's quite a journey back.

It's no problem to drive 700 kilometers from Campbell Port to Adelaide in one day.

I feel like my itinerary is very close to yours. I depart from Melbourne, stay one night each in Apollo Bay, Campbell, and Port Fairy, then spend two nights on Kangaroo Island. After that, I will spend two days in Adelaide before flying back to Melbourne. I will pick up the rental car in Melbourne and return it in Adelaide. The cost of returning the car to a different location is slightly higher, but it saves me from backtracking and is less hassle.

I feel that this plan is feasible.

The Great Ocean Road is narrow in most places and has no mobile phone signal in many areas.