Very nice job. You made a very nice likeness. I have never done a straight grayscale underpainting like this in oils. I have done it once with acrylics, but I ended up using very bright colors in the end so I have no reference for trying to do this using natural colors.
If you like working this way I wonder have you ever tried doing the same thing using pthalo blue and burnt sienna for an underpainting. It gives a really nice effect of warming and cooling certain areas.
I'm following for more.
I have not. But I will try it some time. It's hard sometimes to get out of our comfort zones, but its very important that we as artists do!