Best way to extend "data-only" classes in C# client apps?

Question

I've read through Is it possible to assign a base class object to a derived class reference with an explicit typecast? and want to know if JsonConvert is the best solution in this scenario. I have an existing C# project that makes calls to a web API and returns the data to various clients (Blazor, WinForms, Xamarin, command line, etc.). The problem is that client-specific properties have been added to the data classes. I want to strip out the client-specific code and put them into a derived class. Example call:

    public class ResponseModel<T>
    {
        public long StatusCode { get; set; } = -1;
        public T? Result { get; set; }
        public List<T>? Results { get; set; }
        public string Message { get; set; } = string.Empty;
    }

    public class PromoItem
    {
        public Guid PromoItemId { get; set; }
        public string ItemName { get; set; } 
        public bool IsAvailable { get; set; }
        public int MaxAllowed { get; set; } 
        public string ItemManufacturer { get; set; } 
        public int QtyTotal { get; set; } 
        public int QtyDistributed { get; set; } 
        public Guid CreatedById { get; set; } 
        public string CreatedBy { get; set; }
        public DateTime CreatedOn { get; set; }
        public Guid ModifiedById { get; set; } 
        public string ModifiedBy { get; set; }
        public DateTime? ModifiedOn { get; set; }
    }

ResponseModel<PromoItem> response = await PromoItemService.Get(Guid.Parse("73F166A6-C576-4531-83D5-154B4D16E557"));

Each client app is can do something different with the data. For example, I've stripped this out into a derived class:

    public class PromoItemDTO : PromoItem, INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        public string ItemTypeGlyph
        {
            get
            {
                if (ItemName.Contains("Shirt"))
                {
                    return "\uE8D7";
                }
                else if (ItemName.Contains("Shoes"))
                {
                    return "\uE8AB";
                }
                else
                {
                    return "\uE8D7";
                }
            }
        }

        bool isItemVisible;
        public bool IsItemVisible
        {
            get { return isItemVisible; }
            set
            {
                isItemVisible = value;
                OnPropertyChanged(nameof(IsItemVisible));
            }
        }

        public void OnPropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

So should I be using JsonConvert to convert ResponseModel.Result to a PromoItemDTO object in this scenario? Should I use composition? Or something else?