I can't answer your question, which is faster? depends on test conditions etc.

Direct is maybe a little faster but come at a price of scalability, do some tests. A tiny performance win (if it exists) comes at the cost of a poor design approach IMHO.

Have you tried the BarBackgroundColor property?

@DFangsaur I change some code of your project(Actually, I just changed the from IObservableCollection to ObservableCollection ). Then here is running GIF(NOTE:please add the permission in the settings manually)

I upload your project, you can refer to it.If the reply is helpful, please mark it as answer, it will help other who have similar issue.

I guess you must use Modal style to present the pages. Apple changes the default ModalPresentationStyle from full screen to PageSheet so the second page will overlap the current pages and leave a gap at the top.
You could change this behavior through the custom renderer:

[assembly: ExportRenderer(typeof(ContentPage), typeof(CustomPageRenderer))]
namespace App.iOS
{
    public class CustomPageRenderer : PageRenderer
    {
        public override void WillMoveToParentViewController(UIViewController parent)
        {
            base.WillMoveToParentViewController(parent);

            parent.ModalPresentationStyle = UIModalPresentationStyle.FullScreen;
        }
    }
}

If you are trying to present a page wrapped by a navigation page you need to create a custom renderer for your navigation page too:

[assembly: ExportRenderer(typeof(NavigationPage), typeof(CustomNavigationPageRenderer))]
namespace App.iOS
{
    public class CustomNavigationPageRenderer : NavigationRenderer
    {
        public override void WillMoveToParentViewController(UIViewController parent)
        {
            base.WillMoveToParentViewController(parent);

            parent.ModalPresentationStyle = UIModalPresentationStyle.FullScreen;
        }
    }
}

However, if you update your Forms version to latest this issue has been solved. The UIModalPresentationStyle in Forms has been reversed back to FullScreen.

How long time haven't you updated it .. try to update it to the latest to see if problem persists.

Define a custom navigation container inheriting from the FreshNavigationContainer like:

public class CustomFreshNaviContainer : FreshNavigationContainer
{
    public CustomFreshNaviContainer(Page page) : base(page)
    {
    }

    protected override Page CreateContainerPage(Page page)
    {
        if (page is NavigationPage || page is MasterDetailPage || page is TabbedPage)
            return page;
        return new NavigationPage(page) { BarBackgroundColor = Color.FromRgb(0, 69, 140) };
    }
}

Then you can change the bar background color there.
Do not forget to change the container at the initial time:

var mainNavigation = new CustomFreshNaviContainer(mainPage);

There is a method callled Sum to compute the sum of a sequence of numeric values in Linq , you could pick all the Quantity into a list ,then call this method .

Example code

int sum = tabledata.Select(x => x.Quantity).Sum();