A 28-year-old woman from Mumbai with a 3-year history of nephrogenic diabetes insipidus (confirmed by genetic testing: AVPR2 mutation) presents with recurrent episodes of hypernatremia (Na 155–162 mEq/L) despite high fluid intake and amiloride therapy. Her latest serum creatinine is 1.8 mg/dL (baseline 0.9 mg/dL), and renal ultrasound shows bilateral medullary cystic changes. What is the most appropriate next step in management?