A 68-year-old man with myasthenia gravis presents to the emergency department with acute worsening of ptosis, diplopia, and generalized muscle weakness. He is on pyridostigmine 60 mg QID. Vital signs: BP 140/88 mmHg, HR 92/min, RR 18/min, SpO₂ 94% on room air. On examination, he has increased salivation, lacrimation, and mild bronchospasm. Which is the most appropriate immediate next step in management?