Add text label with semi transparent background to an image using Magick.NET

I have some C# code that adds a simple text overlay with a border and semi-transparent background to an image. It works great, but I'm trying to get an equivalent result using Magick.NET. (The straight C# code drops the XMP tags from the original image, and I haven't found a way to deal with that.) Magick.NET handles the XMP tags well, but I'm having trouble replicating the original output.

Original code follows:

using (Image i = Image.FromStream(stream))
{
  int width = i.Width;
  int height = i.Height;

  using (Graphics graphics =  Graphics.FromImage(i))
  {
    string measureString = "my string";
    Size stringSize = graphics.MeasureString(measureString, stringFont).ToSize();
      
    Point drawLocation = new Point(width - stringSize.Width - 15, height - stringSize.Height - 15);
    Rectangle rect = new Rectangle(drawLocation.X, drawLocation.Y, stringSize.Width, stringSize.Height);
      
    graphics.DrawRectangle(blackPen, rect);
    graphics.FillRectangle(fillBrush, rect);
    graphics.DrawString(measureString, stringFont, Brushes.Yellow, drawLocation);
  }
  i.Save(outputFolder + Path.GetFileName(imgFileName));
}

I cobbled this together based on the Magick.NET examples. This get close to what I'm looking for, but adding the border removes the transparency value, and I'm left with a dark gray background, instead of the transparency.

 var settings = new MagickReadSettings{
                Font = "Calibri",
                FillColor=MagickColors.Yellow,
                StrokeColor=MagickColors.Black,
                TextGravity = Gravity.Center,
                BackgroundColor = new MagickColor("#66666699"),
                BorderColor = MagickColors.Black,
                Height = 250, // height of text box
                Width = 680 // width of text box
            };

using (var image = new MagickImage(inputFile))
{
  using (var caption = new MagickImage($"caption:{myString}", settings))
  {
    //adding this border removes transparency
    // caption.BorderColor = MagickColors.Black;
    // caption.Border(1);

    image.Composite(caption, Gravity.Southeast, CompositeOperator.Over);
    image.Write(outputFile);
  }
}