A 72-year-old woman with a 15-year history of CKD stage 4 (eGFR 18 mL/min/1.73m²) secondary to hypertensive nephrosclerosis presents with nausea, poor appetite, and a 3 kg weight loss over 2 months. She is on lisinopril 10 mg daily, amlodipine 5 mg daily, and atorvastatin. Recent labs: serum creatinine 3.6 mg/dL, potassium 5.8 mEq/L, phosphate 5.2 mg/dL, calcium 7.8 mg/dL, albumin 3.1 g/dL, and hemoglobin 9.2 g/dL. Urinalysis shows 1+ protein, no casts. Which of the following is the MOST important intervention to prevent progression to ESRD and reduce mortality in this patient?