The sample seems too dark to me. Especially after looking at the 3 schemes for the P III.

See my gallery on "The Newcastle - a study in Map Styles"

I found this web site with a series of aerial and satellite imagery. I thought it would be of interest.

There is a problem with Win 10, if you do not have a mouse that is win 10 compliant it will double click. I had a nice Logitech mouse that I really liked, then Win 10, I ended up buying a new mouse, solved all of the problems, except that it does not work as well as the old one did before win 10. :(

Two ways I can think of;

1) trace the contours and just use them as is. Very map like, and may not be what the client had in mind.

2) use the traced contours and more shades of grey to build the gradations. So where you have 3 polygons now ( including the Island base), you would have 6 polygons and 6 shades of grey. I think using the same effects, but some tweaking may be necessary.