Picard's line about the military taking power wouldn't make much sense if Cardassia is not a military dictatorship.
How much can we really read into Picard's opinions? It would be valid to say that the military has taken power in the United States, at any juncture of recent history where the nation has gone to war mainly because the military or its background powers wanted to, and indeed many educated and influential people have expressed that very opinion; yet not a single time has the nation gone to war against its democratic principles of letting the elected government have the final say.
Picard may simply not understand the finer nuances of the Cardassian triumvirate government very well.
Heck, in the beginning of season of 4 of DS9, there was a revolution that overthrew the Central Command and gave power to the civilian government
Which is just a step towards dictatorship and away from a balance of power, in objective terms: it narrows the power base of the government by illegal means.
We have no idea whether the Detapa Council is in any way less dictatorial than the Central Command. We have never heard of anything remotely like an election, or of any sort of a representation system, as regards the Detapa Council.
Sure, Cardassia may be a dictatorship. But just because said dictatorship happens to have a military doesn't make it a military dictatorship; for all we know, it's a bureaucratic dictatorship, ruled with an iron stamp, or a dictatorship of religious fanatics much like Spain under Philip II, just with a fairly straightforward loyalty religion devoid of superfluous supernaturalism.