If u don't like Diego Garcia ... Okay, it's more like Guam island =).
1. SCS islands far aways from China including HaiNan island, YES ! China lack effective air-defense capacity to protect our interests ... Hence ↓
2. Build the long runway & hangars on artificial islands to deploy PLAAF fighters, to protect ships in SCS.
3. Build the deep seaport on artificial islands to deck PLAN FFGs even C.V & Nuclear-subs, to protect ships in SCS.
4. Set HQ-xx anti-aircraft missiles to provide air-defense capacity, to protect ships in SCS.
5. Yes, a resupply base for ships in SCS.
6. Those artificial islands much better & useful for China than deploying a C.V group in SCS, they will be "Super Aircraft Carrier" ... can do much more things in future.