The phrase "at the reins" is commonly used to describe someone who is in control or in charge of a situation. However, there are many synonyms for this expression that can add variety and nuance to your writing or speech. For example, "in the driver's seat" and "calling the shots" both suggest leadership and control. "Holding the keys" and "in command" also convey a sense of authority and responsibility. Meanwhile, "in charge" and "in control" are straightforward and direct synonyms for "at the reins." Consider incorporating some of these synonyms into your writing or speech to add depth and color to your language.