A 68-year-old woman with GOLD stage II COPD (post-bronchodilator FEV1 65% predicted) is reviewed in clinic. She has daily dyspnoea on exertion (mMRC 2) and two moderate exacerbations in the past year requiring oral steroids but no hospitalisation. She currently uses a short-acting bronchodilator as needed. According to the 2023 Canadian Thoracic Society COPD pharmacotherapy guideline, what is the most appropriate next step in maintenance therapy?