A 65-year-old woman with a 5-year history of COPD (post-bronchodilator FEV1 45% predicted) has had three moderate exacerbations requiring oral steroids and antibiotics in the past year despite treatment with a LAMA/LABA inhaler. She continues to smoke. According to the latest Canadian Thoracic Society COPD guideline, what is the most appropriate next pharmacologic step?