public async void ActivateRage()
{
    _isRage = true;
    _bonusChanger.SetBonus(data.GetRageBonus(),bonusInstantiated);
    await Task.Delay(5*1000);
    _bonusChanger.SetBonus(null,bonusInstantiated);
    _isRage = false;
}

//If return type Task you can await this method in place where you call it
public async Task ActivateRageTask()
{
    _isRage = true;
    _bonusChanger.SetBonus(data.GetRageBonus(),bonusInstantiated);
    await Task.Delay(5*1000);
    _bonusChanger.SetBonus(null,bonusInstantiated);
    _isRage = false;
}

private async void ExampleMethod()
{
    ActivateRage();
    Debug.Log("done 1"); //this line will be called without delay
    await ActivateRageTask();
    Debug.Log("done 2"); //this line will be called only after ActivateRageTask finished (after delay 5s).
}