I disagree that the Cardassians and Romulans share too much in common to be interesting alternatives, or that they no longer have a point co-existing in Star Trek. You could argue that the Klingon state is similar, with all the same ideas like "violence is a virtue", what is right for the species is right period, etc, yet they are still interesting. The Romulans are the older foe in Star Trek anyway, and so maybe I just feel they deserve a reverential position as villain. In real life, cultures with political similarity exist, but each is unique in other ways.
The Cardassian Union is predicated around 1). nationalism, 2). worship of family, and 3). worship of sacrifice to the state - "The Never Ending Sacrifice" being their most famous work of literature - they are shown as a people led by Central Command into ignorant sacrifice of their freedoms for the promise of security, because they are impoverished, only to be thrown away like a commodity by their rulers. They also changed as a culture significantly - during TOS, they may have still been in their pre-totalitarian phase, suffering famine, being granted food aid by the Federation - having a completely different culture.
The facade eventually collapsed. Central Command was first ousted by a democratic Detapa Council, then later, replaced by Gul Dukat's coup, in which the hypocrisy of an elite throwing Cardassian life into the grinder was revealed in full, leading to the death of 800 million Cardassians under Dominion rule.
The Romulans in contrast have been a formidable imperial civilization for centuries. One that I sense may not have quite the same combination of paternal ideals as the Cardassian state. The Roman Empire, despite similarities to many modern paternal tyrannies, like Franco's Spain, was still different in details. Like reading a description of Sparta:
Sparta was unique in ancient Greece for its social system and constitution, which configured their entire society to maximize military proficiency at all costs, and completely focused on military training and excellence. Its inhabitants were classified as Spartiates (Spartan citizens, who enjoyed full rights), mothakes (non-Spartan free men raised as Spartans), perioikoi (free residents, literally "dwellers around"), and helots (state-owned serfs, enslaved non-Spartan local population).
On paper, the patrician plantation owners of the Confederate States of America shared similarities with ancient Sparta, but in practice, aside from both being slave owning states, which endows them with constant existential fears of revolt, and many common social parallels of dehumanizing their subjects, the two were probably hugely different in cultural details. The Spartans dined in communal barracks, ate a kind of military ration called "black broth", the state declared war on the helots every Autumn, youngsters had to pass the 'agoge', etc.
Ironically for being Trek's oldest villains we have less idea what their state ideology actually is, ranging from "we are creatures of duty", and being considered for the role of villain in Star Trek III (but being replaced by Klingons because the two were close enough in concept), to the TNG depiction of a culture obsessed with misdirection.
During TNG, they were depicted as being in a period of police-state (perhaps a passing historical period) - but they have also, in TOS, demonstrated a much more Roman social system, around rank/glory.
I can't imagine the Cardassians fielding an auxiliary legion of aliens, but the Romulans deployed Reman auxiliaries in the Dominion War, and may do the same with many other subject species, like the incredibly ethnically diverse Roman Empire.
We should be careful not to associate the Romulans visual design with any sensitive cultural stereotypes, lest we promote outdated racist tropes about asia. We have seen Romulan ambassadors adorned with aristocratic looking clothes in Star Trek V/VI, and there is some ambiguity over whether the empire has a Praetor, Emperor, or both, or changing leaders, but if they did go down the route of making Romulan aristocrats very decadent, a neutral futuristic fashion can be created.
People forget that Rome, and Rome's successors were quite aesthetically diverse at times. Venetian masquerades, Austrian imperial courts. Rome/Byzantium presented all the artistic sophistication of a 2000 year civilization, but people don't learn Latin as standard in grammar schools any more, so public consciousness has shifted away from classical antiquity.