A 68-year-old man with a 45-pack-year smoking history has COPD confirmed by spirometry (post-bronchodilator FEV1/FVC 0.55 and FEV1 55% predicted). He remains short of breath on exertion (mMRC 2) despite using short-acting salbutamol as required. He has had one moderate exacerbation in the past year treated with oral corticosteroids but no hospitalisations. According to the 2023 Canadian Thoracic Society COPD pharmacotherapy guideline, what is the most appropriate next step in inhaler therapy?