A 63-year-old woman with a 35-year history of smoking and chronic cough presents with increasing dyspnoea on exertion. Spirometry shows FEV1 55 percent predicted and FEV1/FVC 0.55 post-bronchodilator. She is already on a long-acting muscarinic antagonist. According to Canadian COPD pharmacotherapy guidance, which next step is most appropriate to reduce exacerbations?