The crowning step can also refer to the final, most important step in achieving some goal. You could also use crowning achievement. "The Olympian's crowning step was winning the gold medal." "The swimmer's crowning achievement was crossing the English Channel."

a step that leads to the crown, which is an excellent step

This could mean when the babies head first appears it is the crowning step of the birth. or Royalty when they are crowned go through a ceremony and part of that is the crowning step