They could have kept up the Roman type motif, which was also short and practical hairdos for the military types we usually see. The civilians in the glimpses on regular Romulus in Unification or non military (Tal-shiar types) should vary a bit more. Senators and higher politicians could have more elaborate hairdos. Different styles and more color variation perhaps.Maybe one day when humans are a truly logical species we'll all cut our hair like that
Which is annoying since they added the bumpy foreheads since the audience was apparently too stupid to know the difference otherwise, but then gave them the same haircut. It makes sense for Vulcans as a simple utilitarian thing but no reason Romulans wouldn't have normal varying haircuts.
Better still would have been to take a page from Balance of Terror and have many of them wear helmets even through TNG.
ENT could have leaned into this but went to TNG for it's uniforms instead of a better reverse engineering/developing the TOS uniforms but that was typical problem they had with continuity (and did it with the Klingons, too) and the perils of prequels and retcons.