I'm not so sure, but the following might work:

...
@State private var myData = MyObservableObject()
...
VStack {
   //display stuff here
}
.task {
   myData.fetchStuff()
}
.onReceive(Just(myData.isReady)) { //<--- here
   if myData.isDoneFetching {
       formatUIData()
   }
}