A 62-year-old male smoker (40 pack-years) presents with progressive dyspnea on exertion and chronic cough productive of mucoid sputum for the past 3 years. He has been diagnosed with COPD (GOLD Stage 2, FEV₁ 55% predicted). On examination: barrel chest, pursed-lip breathing, oxygen saturation 94% on room air. Current medications include salbutamol PRN. Spirometry shows FEV₁/FVC ratio 0.58. He denies exacerbations in the past year. Which is the most appropriate next step in pharmacological management?