You say "a good combination of default notification settings" but I do not know if it is possible to obtain this from a general point of view because precisely, it raises a very personal question: "What does a friend represent for me? " There is no universal answer to this question and therefore no universal solution either.
In my opinion, it would be nice to let everyone answer this question, with perhaps, yes, default options in the administration part, but also and above all, to let choose finesse for each profile would be ideal.