A 42-year-old woman with a 10-year history of asthma and chronic rhinosinusitis presents with recurrent bilateral nasal polyps. She underwent FESS with polyp removal 2 years ago but has had two recurrences since. Endoscopy now shows bilateral polyps with eosinophilic inflammation on biopsy. She is already on high-dose intranasal fluticasone (200 µg BD) and oral montelukast. What is the most appropriate next step in management?