S550: Fifth Thing Fixed
a recall out for a bunch of 2014-20 Mercedes-Benz cars (not mine) for this piece, and Mercedes-Benz apparently has designed a glass replacement panel for those cars.)
The solutions for this paint wear are either vinyl wrap or repaint. I opted for the second path (a bit more money, but likely to look better?), using West Wind Auto, who have done great paint work for me in the past. Now it looks good as new.