One thing that I think makes System Settings much worse than it has to be is how indistinguishable its sidebar icons are. They all have the same shape, some share the same background color or have very similar background colors, their glyph shapes are thin, and the white glyph color has extremely bad contrast with some of the background colors.
If the icons were more akin to 10.6 Snow Leopard sidebar icons[0] it'd give the sidebar items much more substantial visual anchors. Heck even the more muted sidebar icons of 10.7 Lion[1] would be a substantial improvement.
[0]: https://www.versionmuseum.com/images/operating-systems/mac-o...
[1]: https://512pixels.net/wp-content/uploads/2018/08/10-8-Mounta...