A 38-year-old woman from Mumbai with a 10-year history of myasthenia gravis (MG) presents with acute worsening of ptosis, diplopia, and bulbar weakness over 48 hours. She is on pyridostigmine 60 mg QID. Bedside testing shows positive ice pack test. Her serum potassium is 3.2 mEq/L (normal: 3.5–5.0). Arterial blood gas shows pH 7.48, PaCO₂ 32 mmHg. Repetitive nerve stimulation shows >10% decremental response. Which is the most appropriate immediate next step in management?