A 6-year-old girl from rural Maharashtra presents with recurrent episodes of vomiting, lethargy, and behavioral changes triggered by high-protein meals. Serum ammonia is 180 µmol/L. Plasma amino acid analysis shows elevated glutamine and alanine with low arginine. Urine shows elevated citrulline. Genetic testing confirms argininosuccinate lyase (ASL) deficiency. After stabilizing ammonia with benzoate and arginine, what is the most appropriate next step in long-term management?