A 60-year-old man with a 45-pack-year smoking history and chronic bronchitis presents with worsening shortness of breath and productive cough. He has had two exacerbations requiring steroids in the past year. Spirometry shows FEV1 42% predicted. He currently uses LAMA/LABA combination. According to CTS COPD pharmacotherapy guidance, what is the next step in inhaled therapy?