The three Miriams that I've known besides your M in my life have all been Mim or
Mimi, especially around family. I think
Miriam is at the very least the kind of name where there isn't such a familiar nickname that it can almost be seen as a "stand in" for the full name. Like plenty of people go by "
Alex" in all contexts of their life. It isn't seen as a super familiar kind of nickname. My entire family call me Lisey or Lissy but I'd never introduce myself to anyone by that. I have friends who call me that too but again, because it's not a more common nickname, it would seem "too familiar" coming from anyone else.
I don't know if that makes sense but it's how I think of
Miriam. The first name is so uncommon that there isn't a clear "go to" nickname that feels everyday enough for a stranger to just jump to and use.