using System;

using System.Threading.Tasks;

using System.Linq;

public class Program

{

    public static async Task Main()

    {

        var myStuff = new string[] {"A:1", "A:2", "A:3", "B:1", "B:2", "C:1", "C:2", "C:3", "C:4"};

        var groups = myStuff.GroupBy(s => s.Split(':')[0]);

        await Task.WhenAll( 

            groups.Select(async d => {

                // need an array or list instead of IGrouping enumerable for index support

                var data = d.ToList(); 

                await Task.WhenAll(data[..^1].Select(async item => await SomeMethodAsync(item)));

                await SomeMethodAsync(data[^1]);

            })

        );

    }

    static Random r = new Random();

    public static async Task SomeMethodAsync(string s)

    {

        var delay = r.Next(3000);

        await Task.Delay(delay);

        Console.WriteLine(s);

    }

}