Weren't Capt. Pike's quarters in "The Cage" significantly different from Kirk's? I don't recall if they were ever given a location, but the impression I had was that Deck 2 housed Pike's quarters and the briefing room shown in that story. One thing is for sure: the set (or sets?) used for Pike's room and the briefing room are significantly different than Kirk's cabin and the TOS-proper (post-"Where No Man…") Briefing Room sets.
Cross-comparing this to Picard's Ready Room on the Enterprise-D and to Archer's Ready Room on the NX-01 Enterprise, it seems to have gone full circle. Picard's ready room is like a combination office and mini-briefing room. (Seen as such in "The Neutral Zone") Other than that, the combination of a replicator and the (implied) off-set access to what seemed to be an executive washroom/head, seemed to give Picard's facility very limited function, like a "Captain's Break Room / Reading Room".
Archer's just-off-the-bridge facility seemed much more logical. It appeared to be a mini-office, plus a bedroom-at-the-Bridge. So the makers of ENT morphed the Picard Ready Room concept, merging it with Pike's.
One question on my mind: does anyone here think Kirk needed a Ready Room just off the Bridge? Would it have had useful dramatic value for TOS if such a set had been created?
One important fact that seems to be overlooked: all STAR TREK starship facilities are paperless, or very nearly so. They use padds, which look like iPads or Kindles. They also use desktop display terminals and other viewscreens all over the ship. With everything computerized, why is a dedicated just-off-the-bridge office necessary? TNG made use of it (and it seemed cozier than their Observation Lounge), but was it necessary? Couldn't the captain set up an office in his quarters, or in a briefing room, as we often saw Kirk do? If the Main Bridge on a Constitution-class starship were either damaged or undergoing refit/repair, and that part of the ship were rendered inaccessible, Kirk's quarters on Deck 5 would be a safe distance away and actually closer than the (implied) locations of Auxiliary Control (in either the underside of the saucer or in the neck or secondary hull).