Ok, since no one else has mentioned it, I’m going to mention how the Typhon Pact formed in the Relaunch novelverse.
Different cataclysm, but divided remnants of the Romulans joined up with the Tholians, Breens and assorted others to establish a competitor to the Federation, which was seen as failing its perimeter members planets and species.
The book that goes deep into the heart of Breen space and culture is called ‘Zero Sum Game’. It’s Bashir and an old colleague sucked into spy stuff. The most awesome Captain Ezri Dax makes a significant appearance too.