I have also found that a flatter spatula angle (perhaps 20 degrees) works better for smoothing than a more perpendicular angle (90 degrees).
Ah, that explains why my final solution worked: I used one of those Wilton flat triangle shaped profiling tools to put rows of equally space rows around the edge. I had to hold it flat to avoid removing too much icing. If anything, the frosting was too thick; maybe because I added the Bill quantity of chocolate, but didn’t reduce the egg whites.
As a side note, I’ve determined that Chocolate buttercreams are very unsatisfying, at least ones where the eggs get whipped. Way too light. Flo Braker has an interesting version she calls “Sabayon Buttercream” that I may try next.