A 58-year-old man presents to his GP with cough and shortness of breath. He has smoked 40 pack-years. Spirometry shows FEV₁/FVC 0.55 and post-bronchodilator FEV₁ 45% predicted. He has had two exacerbations requiring oral Prednisolone and antibiotics in the last year, but no hospital admissions. He is currently on a long-acting muscarinic antagonist (LAMA) alone. According to COPD-X and Therapeutic Guidelines, which is the most appropriate next step in pharmacological management?