I have a strong opinion. 
Don’t use shared inbox unless you want to support instances of tens of thousands of users, and even then there are other ways to scale. As I understand, it was only introduced so that Gargron could easily scale his flagship instance.
It’s the only ActivityPub feature that breaks the Actor model. If you don’t give Actors their own inbox, then you’re doubling down on breaking the Actor model. I don’t recommend breaking the actor model.
Edit to add: convention is some software supports shared inbox, but all software supports the Actor model.