The official name for my intergalactic, utopia-aspiring empire is the Empire of Light. They avoided an "ethnic" name (unlike their defunct predecessor the Terran Empire) to show their commitment to the faith that united all humanity after the Serpent Wars, the Church of the Eternal Light. (It is estimated that ninety-eight percent of the human race died during the Serpent Wars, a conflict between a cult ran by a demonic entity and those who resisted and chose the way of light beneath a messianic figure known as the Mother of Mages. Her distant descendant, Zecharias led humanity to the stars where they created a new society based upon altruism and reverence for nature.) This means the Empire is more of a religious institution similar to the Roman Catholic Church, though the Imperial Family has some political authority though most decisions are made by a democratically elected Senate and religious tolerance is part of the official state doctrine, except for Demon Worshippers because, well, one doesn't tolerate those trying to summon entities wanting to eat the universe. Anyway, is the name cumbersome?
All that, and the OP worries about the name! I'd suggest it might use more words than it needs to. Asimov got first dibs on "The Empire," and Games Workshop got "The Imperium," and the "of Light" isn't carrying much information to the reader, but it might be repeated hundreds of times over the course of an space opera. Maybe 0.5% of the novel's wordcount could end up being the "of Light"s on the end of Empire of Light. I'd suggest to come up with something short and catchy that sums up that these are the goodies. e.g. I'd pick out that it's founded by the Mother of Mages, and the 3 magi in the christmas story were Melchior, Balthazar and Caspar, and the first two are already overused in space operas so I arrive at: The Casparian Empire. From Google it turns out someone has already used that in a hobby game, but it's no longer active and they can't afford lawyers.
I cheat a bit and have the characters call it "the Empire" while their enemies have a horde. Luckily, there doesn't seem to be any other empires to rival them!
You could call it The Illuminati. That is short, of course, for The Glorious Universal Empire of the Resurrection of the Light of Lucia, the Mother of Mages.
I’d go the other way, considering that other franchises have The Empire and maybe call them The Light. It gives them the idea of being a force for good, cutting through the world and I’m eradicating darkness. Whether that’s true doesn’t matter. No empire called itself The Baddies (although I’d be more imperialist if they were that bold).
The name is not cumbersome, but I would eye every nation that uses this name unironically with utmost suspicion.
"Shh...don't speak of such things the Light will hear." No the name isn't cumbersome, a bit generic though. And I wouldn't use The Illuminati, since that has been a long defunct secret society that used to share knowledge and stuff, before it got hijacked by the conspiracy theorists for their wacko ideas cause it sounds spoopy. However, it does have a real world sound to it, that it would be plausible that something like that could arise as a commanding faction in the future. It's more interesting to learn what they do in some detail, rather than overthinking what to call them. In a way it has a sinister quality to it, but to anyone not in the know may or may not pick up on that darker implication behind such a benign sounding Empire.
Anyone who suggests The Glorious Universal Empire of the Resurrection of the Light of Lucia, the Mother of Mages maybe less than serious when suggesting The Illuminati.
