A 68-year-old man with type 2 diabetes and chronic kidney disease (eGFR 35 mL/min/1.73 m²) presents with widespread crusted plaques, hyperkeratosis, and nail dystrophy. Microscopy confirms *Sarcoptes scabiei*. He has failed two courses of topical permethrin (5%) applied 1 week apart. His wife is unaffected. What is the most appropriate next step in management?