The Ready Room

The use of the term "Ready room" as a place where the Admiral or Commander of a ship can conduct briefings or business while next to the bridge appears to originate with "Star Trek, the Next Generation" although the term has been used at sea on US Navy aircraft carriers, to describe a room where briefings for Aircrew are held. 

On a major Royal Navy vessel the captain's office will be referred to as the "Captain's day cabin" which will often be located as part of a suite of living and working quarters at the stern of the ship. The captain of an RN warship of any size will usually also have a "Captain's Sea Cabin" located very near to the Bridge so he can be called there quickly to take charge in the event of any emergency.

On a starship the size of an Imperial Star Destroyer such as Grand Admiral Thrawn's flagship, the ISS Chimaera, space is not at a premium, even less than on a Galaxy class Federation starship such as the Enterprise D.

But the time required to respond to sudden threats is even more at a premium. Thrawn does need to be able to get to his Flag Bridge quickly.

Throughout his career, Grand Admiral Thrawn has been noted for his willingness to use good ideas from any source. While a Senior Captain in the Chiss Expansionary Defence fleet, Thrawn bought back a Shield generator from the mission on which he helped General Skywalker rescue Senator Padme Amidala. 

We have learned as recently as May 2024 that the fighter craft design most associated with Thrawn, the Tie Defender, was developed by a Nightsister from Dathomir, Morgan Elspeth. She had originally offered this design to an imperial committee chaired by Moff Isdain, who was too stupid to see the war-winning strategic value to the Empire of a fighter with shields and a hyperdrive. Thrawn, however saw the value of this ship design at once, and recruited Morgan Elspeth to build the Tie Defender for the empire on Lothal.  

So the little matter that the idea of a captain or admiral's ready room to hold briefing near the bridge comes from a different series set in another galaxy would certainly not stop the Grand Admiral from adopting such a useful idea.