The CSS is complex indeed. Moreover, the page renders the Shorts widgets after the page has completed loading, so JavaScript-based solutions need to exercise a little care in ensuring that it can hide the distracting elements after the page has loaded. Despite the complexity, I've found that just hiding all elements with the 'ytd-rich-shelf-renderer' class just works fine. For example:

  .ytd-rich-shelf-renderer { display: none }

This single line of CSS seems to just work fine for me.