This particulcase seems ambiguous to me. It's perfectly acceptable in english to say "I want to be bishop" just like someone might say "I want to be king/queen"
I feel like a similar sentence: "My grandson wants to be president" would translate to "Meu neto quer ser presidente"
Maybe this is because bishop/king/queen/president are not only professions, but are also titles?
I also feel like I'm splitting hairs, so I apologize for that.