A 29-year-old man with generalized anxiety disorder was started on sertraline 50 mg daily 6 weeks ago. At follow-up, his GAD-7 score has decreased from 16 to 12, and he reports some improvement in worry but continues to have significant muscle tension, difficulty concentrating, and restless sleep. He tolerates the medication well with no side effects. Using measurement-based care principles, what is the most appropriate next step?