I feel like the whole point of mercenaries specifically in these sorts of games is that they exist to let you fill out a class you don't already have on hand. They're not meant to be characters in and of themeslves and I don't think BG3 is structured in a way that they would be beneficial. They're meant to be purely functional, and giving them personality to the degree you suggest and making them untrustworthy would defeat the purpose.
Also while more companions would be better, I don't think think Larian can support so many companions, and I feel like if they end up becoming only empty suits with a couple repeating lines they say over and over, then that would be detrimental to them as characters.
