Or would it be better to hide the truth for a time, allow peace to take root while you gather evidence, then expose it?
The error there is that when will be enough time, or that the next reason to hide the truth is "Let the past rest" Saying that things are calm and stable now, and to bring up the past would just open old wounds.
Isn't that the whole theme of the novel? You've got the Cardassians trying to become their apparent Federation ideal, while in reality, the Federation is, and always has been, an imperfect organization. The shadows of the struggle between the True Way and Obsidian Order were paralleled on the Federation side. You could say that Picard went the route of the True Way, attempting to maintain order first, above all else.
And yes, while the choice is a bitter pill to swallow, it really was: Will you blow up the quadrant today, or strive for peace above all else? Even truth. What good is the truth if it spins the quadrant into war?