A 28-year-old woman with biopsy-proven lupus nephritis (Class IV) presents with nephrotic syndrome (proteinuria 4.5 g/day, serum albumin 2.8 g/dL, eGFR 45 mL/min/1.73m²) and active urinary sediment. ANA is positive, anti-dsDNA titre is elevated, and C3 is low. She is currently on hydroxychloroquine and low-dose prednisolone 10 mg/day. What is the most appropriate next step in management?