How can one balance between the virtue of calling to Allah and the obligation of obeying parents, if they refuse it out of fear that the caller may be harmed?

Inviting to Allah is among the best of deeds, but it must be done with insight and knowledge so that you do not corrupt from where you think you are reforming. There are many means of dawah in our time, so choose from them what will not anger your parents or arouse their fears, while achieving your goal of spreading good. And know that whoever seeks good, Allah will grant it to him. Combining parental رضا (pleasure) and dawah is achievable, Allah willing.