I wonder if you could just step back a few feet from her when she gets into your personal space? These days that would not be considered weird or rude. You could turn slightly away from her, step back and if she persists you could tell her you are busy and leave the conversation. I have always struggled to speak up and one thing I’ve found is that sometimes walking away from a situation is a better option than trying to say something. In this case, you really shouldn’t need to say something because social distancing is technically what we are expected to be doing right now.