A 68-year-old woman with Parkinson disease of 8 years' duration, currently on levodopa 750 mg/day with carbidopa, presents with involuntary writhing movements of her limbs and trunk that occur 2–3 hours after each dose of medication. She also reports periods of sudden immobility ('off' episodes) lasting 15–20 minutes between doses. Her Montreal Cognitive Assessment is 26/30. What is the most appropriate next step in her management?