Also, just to add, Coruscant holds tremendous meaning to the Imperial identity and ideology. Why, when they surround it on all sides, would they allow the New Republic to have it?
Because the Imperial rump government
doesn't surround Coruscant on all sides.
As I understand it, the Galactic Concordance peace treaty was basically a "status quo" peace deal
except for Coruscant switching from Imperial control to a nominally-neutral single-system government of its own. The Empire and New Republic ceased all offensive activity against each other, so worlds that were still by the Empire stayed Imperial, worlds that had already been liberated by the Rebels became New Republic, and worlds that were actively contested at the time held elections to pick a side later on.
This means the Imperial rump government (which unlike EU Legends isn't officially named anything) controls an unstated number of worlds in the Core, Colonies, and Inner Rim sectors of the Galaxy, canonically including Commenor, Humbarine, Tinnel IV, Rendili, and Denon, while the New Republic controls the rest of former Old Republic territory, i.e. the rest of the Core, Colonies, and Inner Rim, the entire Mid-Rim, and the entire Outer Rim.
The Imperial factions like Moff Gideon's, which didn't accept the peace deal, mostly retreated to the Outer Rim because the New Republic's actual hold there is even more tenuous than that of the Old Republic. A
specific set of very hardline Imperials retreated even further than that into the Unknown Regions, where they formed the faction that deccades later came back into New Republic territory as the First Order.
In other words: most anti-peace Imperials became their own pocket factions in the Outer Rim. The First Order are the equivalent of the Clans, they fucked ALL the way out into unexplored territory in order to give themselves more time to rebuild and come back with better technology and shit. The 90% disarmament of the New Republic was based on, "The inner systems Imperial rump is keeping to the treaty and the 10% of our current fleet that we're keeping
is more than they have left, we don't give a shit about the riffraff in the Outer Rim, and we
don't know about the First Order hardliners that went further out."