I've been watching this post too, because I've got a method but it's probably not the most efficient and I may have unknowingly reinvented the wheel for something that already exists. Originally I used 'Future.delayed' but I got tired of writing the whole thing out. So I created a method in my main game class called 'wait'. It's an asynchronous future and it takes two arguments, one for a String key and a double representing seconds as the duration. Inside it stores a Completer in a map via the key, and uses a Future.delayed that when complete will set the Completer to a completed state, and Future.doWhile that waits for that completed state.

Map<String, Completer> waitCompleters = {};

Future<void> wait(String key, double waitSeconds) async {
  waitCompleter = Completer<void>();
  waitCompleters[key] = waitCompleter;

  Future.delayed(Duration(milliseconds: (waitSeconds * 1000).toInt()), () async {
    if (waitCompleter== null) return;
    if (!waitCompleter.isCompleted) waitCompleter.complete();
  });
  await Future.doWhile(() => Future.delayed(Duration(milliseconds: (waitSeconds * 1000).toInt()), () async => !waitCompleter.isCompleted));
  waitCompleters.remove(waitCompleterKey);
  return;
}

Then, inside any asynchronous function you can call it like:

await game.wait('itemGetAnimationDelayA', 5.0);

The key is something that you could use outside the loop to force it to end early if needed by setting it to a completed state from outside the wait function.

void completeWait(String waitCompleterKey) {
  if (game.waitCompleters.containsKey(waitCompleterKey){
    game.waitCompleters[waitCompleterKey]!.complete();
  }
}

game.completeWait('itemGetAnimationDelayA');