A 68-year-old man with stable chronic obstructive pulmonary disease (post-bronchodilator FEV1 55% predicted) remains breathless on exertion despite optimal use of a long-acting muscarinic antagonist inhaler. He has had two moderate exacerbations in the last year. According to the 2023 Canadian Thoracic Society COPD pharmacotherapy guideline, what is the most appropriate next step?