WPF loading data asynchronously and contextmenu save as json in mvvm

Install-Package Prism.Wpf;
Install-Package Prism.DryIOC;
Install-Package System.Text.Json;

<prism:PrismApplication x:Class="WpfApp36.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:WpfApp36"
             xmlns:prism="http://prismlibrary.com/">
    <prism:PrismApplication.Resources>
        
    </prism:PrismApplication.Resources>
</prism:PrismApplication>
using System.Configuration;
using System.Data;
using System.Windows;

namespace WpfApp36
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : PrismApplication
    {
        protected override Window CreateShell()
        {
            return Container.Resolve<MainWindow>();
        }

        protected override void RegisterTypes(IContainerRegistry containerRegistry)
        {
            containerRegistry.RegisterSingleton<IIDService, IDService>();
            containerRegistry.RegisterSingleton<IISBNService,ISBNService>();
            containerRegistry.RegisterSingleton<INameService, NameService>();
            containerRegistry.Register<MainWindow>();
            containerRegistry.Register<MainVM>();
        }
    }

}
<Window x:Class="WpfApp36.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp36"
        mc:Ignorable="d"
        WindowState="Maximized"
        Title="{Binding MainTitle}" Height="450" Width="800">
    <Grid>
        <DataGrid ItemsSource="{Binding BooksCollection,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                  VirtualizingPanel.IsVirtualizing="True"
                  VirtualizingPanel.CacheLengthUnit="Item"
                  VirtualizingPanel.CacheLength="2,2"
                  VirtualizingPanel.VirtualizationMode="Recycling"
                  ScrollViewer.IsDeferredScrollingEnabled="True"
                  ScrollViewer.CanContentScroll="True"
                  FontSize="30"
                  AutoGenerateColumns="True"
                  CanUserAddRows="False"
                  IsReadOnly="True">
            <DataGrid.ContextMenu>
                <ContextMenu>
                    <MenuItem Header="Save As Json File"
                              Command="{Binding SaveAsJsonFileCommand}"
                              CommandParameter="{Binding RelativeSource={RelativeSource AncestorType=ContextMenu},Path=PlacementTarget}"/>
                </ContextMenu>
            </DataGrid.ContextMenu>
        </DataGrid>
    </Grid>
</Window>
using Microsoft.Win32;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.IO;
using System.Runtime.CompilerServices;
using System.Text;
using System.Text.Json;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;

namespace WpfApp36
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow(MainVM vm)
        {
            InitializeComponent();
            this.DataContext = vm;
            this.Loaded += async (s, e) =>
            {
                await vm.InitBooksCollection();
            };
        }
    }

    public class MainVM : INotifyPropertyChanged
    {
        private IIDService idService;
        private INameService nameService;
        private IISBNService isbnService;
        public MainVM(IIDService idServiceValue, INameService nameServiceValue, IISBNService isbnServiceValue)
        {
            idService = idServiceValue;
            nameService = nameServiceValue;
            isbnService = isbnServiceValue;
            SaveAsJsonFileCommand = new DelCommand(SaveAsJsonFileCommandExecuted);
        }

        private void SaveAsJsonFileCommandExecuted(object? obj)
        {
            try
            {
                SaveFileDialog dialog = new SaveFileDialog();
                dialog.Filter = "Json File|*.json";
                dialog.FileName = $"Json_{DateTime.Now.ToString("yyyyMMddHHmmssffff")}";
                if (dialog.ShowDialog() == true)
                {
                    var dg = obj as DataGrid;
                    var items = dg.Items.Cast<Book>()?.ToList();
                    var jsonStr = JsonSerializer.Serialize(items,new JsonSerializerOptions()
                    {
                        WriteIndented = true
                    });

                    using (StreamWriter writer = new StreamWriter(dialog.FileName, true, Encoding.UTF8))
                    {
                        writer.WriteLine(jsonStr);
                    }
                    MessageBox.Show($"Saved in {dialog.FileName}");
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);    
            }                      
        }

        public async Task InitBooksCollection()
        {
            BooksCollection = new ObservableCollection<Book>();
            List<Book> booksList = new List<Book>();

            for (int i = 1; i < 3000001; i++)
            {
                booksList.Add(new Book()
                {
                    Id = idService.GetID(),
                    Name = nameService.GetName(),
                    ISBN = isbnService.GetISBN(),
                    Comment = $"Comment_{i}",
                    Content = $"Content_{i}",
                    Summary = $"Summary_{i}",
                    Title = $"Title_{i}",
                    Topic = $"Topic_{i}"
                });

                if (i % 1000000 == 0)
                {
                    var tempList = booksList.ToList();
                    booksList.Clear();
                    await Application.Current.Dispatcher.InvokeAsync(() =>
                    {
                        foreach (var book in tempList)
                        {
                            BooksCollection.Add(book);
                        }
                        MainTitle = $"Now is {DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss")}," +
                        $"loaded {BooksCollection.Count} items";
                    }, System.Windows.Threading.DispatcherPriority.Background);
                }
            }
        }

        public ICommand SaveAsJsonFileCommand { get; set;  }

        public event PropertyChangedEventHandler? PropertyChanged;
        private void OnPropertyChanged([CallerMemberName] string propertyName = "")
        {
            var handler = PropertyChanged;
            if (handler != null)
            {
                handler?.Invoke(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        private string mainTitle;
        public string MainTitle
        {
            get
            {
                return mainTitle;
            }
            set
            {
                if (value != mainTitle)
                {
                    mainTitle = value;
                    OnPropertyChanged();
                }
            }
        }

        private ObservableCollection<Book> booksCollection;
        public ObservableCollection<Book> BooksCollection
        {
            get
            {
                return booksCollection;
            }
            set
            {
                if (value != booksCollection)
                {
                    booksCollection = value;
                    OnPropertyChanged();
                }
            }
        }
    }


    public class DelCommand : ICommand
    {
        private Action<object?> execute;
        private Predicate<object?> canExecute;
        public DelCommand(Action<object?> executeValue, Predicate<object?> canExecuteValue = null)
        {
            execute = executeValue;
            canExecute = canExecuteValue;
        }

        public event EventHandler? CanExecuteChanged
        {
            add
            {
                CommandManager.RequerySuggested += value;
            }
            remove
            {
                CommandManager.RequerySuggested -= value;
            }
        }

        public bool CanExecute(object? parameter)
        {
            return canExecute == null ? true : canExecute(parameter);
        }

        public void Execute(object? parameter)
        {
            execute(parameter);
        }
    }

    public interface IIDService
    {
        int GetID();
    }

    public class IDService : IIDService
    {
        int id = 0;
        public int GetID()
        {
            return Interlocked.Increment(ref id);
        }
    }

    public interface INameService
    {
        string GetName();
    }

    public class NameService : INameService
    {
        int idx = 0;

        public string GetName()
        {
            return $"Name_{Interlocked.Increment(ref idx)}";
        }
    }

    public interface IISBNService
    {
        string GetISBN();
    }

    public class ISBNService : IISBNService
    {
        int idx = 0;
        public string GetISBN()
        {
            return $"ISBN_{Interlocked.Increment(ref idx)}_{Guid.NewGuid():N}";
        }
    }

    public class Book
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string ISBN { get; set; }
        public string Comment { get; set; }
        public string Content { get; set; }
        public string Summary { get; set; }
        public string Title { get; set; }
        public string Topic { get; set; }
    }
}