A 68-year-old former smoker with moderate chronic obstructive pulmonary disease (FEV₁ 55% predicted) reports daily dyspnoea on exertion and uses a short-acting salbutamol inhaler several times a day. He has had one exacerbation requiring oral corticosteroids in the last year. According to the COPD-X guideline, what is the most appropriate next step in pharmacological maintenance therapy?