How would you advise a pre-menopausal, HR-, HER2+ BC with treated brain metastases and NED while on trastuzumab/pertuzumab who wants to conceive?

I would advise against her carrying the pregnancy. Trastuzumab is associated with a high risk of oligohydraminos so she would need to come off therapy for about 10 months. That could cause any dormant micromets to get out of control during pregnancy and jeopardize the baby and the mother's health.