Okay at the moment I am contemplating two things about my mindflayer realm of Stûza.

Firstly, I am imagining they will important some species with them from their original semi-underground territories which they fled from the more dangerous Hegemon which controls the larger other half of Illithid civilisation, or change some of the locals.
I am thinking they would probably take a large section of Koatoa, Gith, goblinoids and maybe some frogmen esque things along with them and that eventually the Dwarves kingdoms who they take would eventually morph into Duregar (although the Dwarves here are a weird fusion of the TES Dwarves, and the Dwarf Fortress ones ie they look kinda Assyrian).

I also mentioned as well that they would probably lack the normal squidlike appearance of  mindflayers but probably would be a colonial organism composed of several specialised parts that form gestalt genetic consciousnesses. The core of them will still require their usual parasites to form but they look much more Hunter like, with their basic forms relying upon metal plates to help keep form (Which may just be another part of their colonial organism). I was furthermore thinking that their intelligence deserves from the psionic nature of these things. Elder brains (which I am just going to call either nodes, Jams/Rajas or Sophys) would be cocooned masses of intellects consumed by it, and would have a protective shell which would function as its body and probably floats with internal gas sacs.

Politically they probably heavily rely upon their psionics to influence the local population and largely characterise themselves to their population as a caste of perfect rulers who improve upon their forms. Internally I suspect while some customs vary they probably organise themselves along the lines of Platonism, with segregated castes of "servers," "agents" and "masters" depending on the nature of the creature. The masters being the psionic adepts who would serve as Sophys later in life. Generally works on a consensus based system which favours the insight of elder and higher castes. The eight Kings of the realm would be a mixture of Massive ancient Sophys and liches (and one draconid) from the original conquest and would share influence over their realms. They would lack a notion of territory and probably think more in terms of personal ownership as well as lacking ideas of normal monarchy. They also probably are the largest slave drivers in the area and probably kickstarted it even more when they arrived.

Economically while reliant upon other races to ensure the that their system works, they control some of the best agricultural land on the continent and so remain a net exporter of often rare produce. This is further helped by the extensive logistical infrastructure put in place through the large network of roads patrolled by gith and hobgoblin slave soldiers and scorpion folk. Superior in quality to nearly all their competitors in their smooth regularity and peacefulness These were made to capitalise on their unique control of the mountain chokepoint which is the only way for most of the continent to access the lands to the west without dealing with the stormy shallow waters of the north west. The natural deep harbours form a unique advantage and they are all but entirely defensible with regular food shipments and being impossible to approach other than up front.

King Ghadra the draconic maintains great influence over the islands through a system of tributaries he maintains through his immense personal strength and Draconic form aweing the locals into submission. While freer than the rest of the empire, their strategic use as a way of controlling trade in the ocean and providing natural sailing paths to the nearby continent means that they are carefully controlled.