A few remarks:
It's actually "over five centuries" prior to DS9, not 400, that the triumvirate of CC, OO and DC was founded. Or at least the episode "Defiant" has Dukat saying that this system has worked more or less smoothly for that length of time; perhaps it is actually much older, but suffered its last major breakdown around the time of Bismarck.
And if "when the military dictatorship actually began" might be debatable in the Trek universe, the very idea of whether there existed
a military dictatorship on Cardassia at all is less than clear as well. Sure, Central Command had the ability to conduct military operations pretty much as it pleased. But the Detapa Council still apparently had plenty of power, and episodes like "Cardassians" indicate that Central Command had to yield to it and withdraw from Bajor after suffering too many humiliating defeats. (Of course, "Cardassians" did not yet use that exact terminology, but it was still a case of the civilians telling the military to get out of Bajor.)