For most of the major cruise lines these days, the price you pay will not vary between booking direct with the cruise line or with a TA, because most of the cruise lines prohibit most discounting. There are exceptions, like if a TA has an existing group booking - and a good one should tell you if they are booking you into group space vs. individually.

These days, typically how TAs distinguish themselves monetarily is offering "free" perks like onboard credit, free gratuities, reduced deposit, etc.

For your group, the reason to go with a full service TA is because you don't know the differences between cruise lines, ships, cabins, etc. A decent full service TA is going to be able to guide you to the right cruise line, ship, combination of cabins etc. You don't really have a large enough group to get a group booking discount on most lines (as that typically requires a minimum of 8 cabins on most lines).

And honestly, if it were just you and your wife, I would say book direct through the cruise line, as you obviously know how to handle travel issues on your own, etc. But an individual trip is a lot different than handling a group on a cruise. The biggest advantage for you is instead of you being the one everybody turns to for every question (which you won't necessarily know yet because you haven't been on a cruise), you just tell everybody to talk to the TA.