I actually would advocate for adding diplomatic suites to the Federation. Why?
Because for quite a while here, it's going to be the biggest, most impressive, best protected, and generally shiniest ship in Starfleet. Enough guns to conduct gunboat diplomacy on Klingons (they like it better that way anyhow). Enough interior space to build a VIP suite for a whale. Enough Max Cruise to make executive couriers look like slugs. Enough shielding that you can conduct peace talks in the middle of a warzone. This ship is going to be the poster-girl battleship of Starfleet for decades.
Additionally, we're not going to get too many of these. Just a simple fact of its size and cost, we'll end up with a dozen or so most likely. Not enough to handle other tasks all over the Federation. But diplomatic duties and VIP transport don't need too many ships dedicated to them, particularly when they're speedy.
Doesn't mean we can't stuff other things in it, but I do really think this is the best spot we're going to get for a diplomatic cruiser for a good while, particularly as we aren't getting Explorer designs until the Excaliburs get retired from the sounds of their epiloge.