Waiting time for refuelling in CAG highly depends on the traffic situation.
If you try to fuel during the peak days (FR/SAT/SUN), you might indeed wait for some time. Generally you can get all you need in CAG, people are really helpful (itīs Sardegna! I know, Iīm based here).
My favorite from the small airfields on the island is Tortoli / Arbatax.
Spectacular landscape, RWY threshold directly on the waterfront, tiny little airport with a very limited amount of commercial (charter) traffic.
Worth a view, try cancelling IFR around Olbia and fly VFR along the coastline - you will remember!
IFR is possible there (cloudbreak procedure with the local beacon). Donīt know about customs and avgas.