I have never run
Traveller in the Imperium, so I sympathise.
I like the idea, but I am not so sure about the pocket empires. It appeals more to me to have systems with multiple habitats under central control (main planet (or two), a couple of mining colonies, O'Neill colonies, asteroid settlements, etc).
I would have two major system-states. One is really industrialized with an enormous commercial fleet. The megacorps keep the government starved, the Army is small and amateurish, the Navy/Marines small but professional, and the Scouts are small but formidable.[Athens Analog]
The other big system-state may even have two habitable planets, but is resource poor. The Army is formidable and probably got its start when the small planet conquored the big one, but they can't afford many starships. They bully the nearby systems and may have conquored one or two poor ones, but lack of resources keep them a local threat. The Army is feudal/hereditary supported by a serf class. Not so much a military dictatorship as a military oligarchy.[Sparta Analog]
Besides the other system-states in this cluster you can have other pocket empires nearby. A couple of low-tech (barely interstellar) regions with cultural commonality with the City States, sort of backwoods cousins [Analogs to Macedonia and Thrace]
More distant pocket empires are more alien (culturally or literally) as analogs of Persia, Egypt, Carthage, Sarmatia and Rome. Maybe even Nubia, Hindustan, Tocharia or China. You can even have human colonies on the perifery of these Empires, like the Ionian city states, Syracuse, the Crimean colonies.
And of course there is a lot of fun to be had disguising the source of this universe, filing off the serial numbers so to speak.