A 42-year-old female with acute respiratory distress syndrome (ARDS) secondary to pneumonia is on day 3 of mechanical ventilation. Current settings: Assist-Control (AC) mode, tidal volume 6 mL/kg IBW, respiratory rate 18/min, PEEP 12 cm H₂O, FiO₂ 0.8. ABG shows pH 7.38, PaCO₂ 42 mmHg, PaO₂ 88 mmHg. CXR shows persistent bilateral infiltrates. The patient is breathing synchronously with the ventilator and has a respiratory drive. Over the past 6 hours, the FiO₂ requirement has decreased from 1.0 to 0.8. What is the most appropriate next step in management?