Is there a way to await a Console.ReadLineAsync and another Task in parallel with WaitAny()?

Console.In.ReadLineAsync is actually a pseudo asynchronous method, it still blocks the current thread just like Console.ReadLine causing Task.WaitAny to not execute at all.

You can create such a task to avoid it blocking the thread:

static Task TaskInput()
    => Task.Run(Console.ReadLine);

Besides, you code may still have an error. You should want only one ReadLine to be executed at the same time, but now each loop will result in a wait for Enter +1. This input task needs to be recreated at least after it is completed:

Task inputTask = TaskInput();
while (!quit) {
    string? inputResult = null;
    var completedTask = await Task.WhenAny(inputTask, backgroundTcs.Task);
    if (completedTask == inputTask)
    {
        inputResult = inputTask.Result;
        inputTask = TaskInput();
    }
    Console.WriteLine($"Status: {inputTask.Status}, {backgroundTcs.Task.Status}"); 
    // do something with either of the tasks, depending on their IsCompleted properties
    if (inputResult != null)
    {
        quit = DispatchDebuggerCommand(inputResult);
    }
}