I looked at your FCStd file, and the shape looks a bit rough. It appears that you have the Tesselation at a coarse setting.
Look at Edit >> Preferences >> Part Design >> Shape view.
The "Maximum deviation . . . " in your file is set to 0.5%, which is pretty coarse. I keep mine at the minimum, 0.01%. It slows the computation a bit, but not a problem for moderate size files.
As an example, I added a circular object inside your ring. You can see the difference in smoothness. The original is magenta, and my addition is green.