A 2-month-old infant born at term in Delhi presents with poor feeding, failure to thrive, and progressive cyanosis since age 4 weeks. The parents report that the baby becomes more cyanotic during crying and feeding. On examination, the infant is severely cyanotic (SpO₂ 65% on room air), with a single loud S2 and a systolic ejection murmur at the left upper sternal border. Chest X-ray shows a 'boot-shaped' heart with markedly reduced pulmonary vascular markings. Echocardiography confirms tetralogy of Fallot with severe right ventricular outflow tract obstruction and a large VSD. The infant is currently on propranolol prophylaxis but continues to have recurrent cyanotic spells. What is the most appropriate next step in management?