that something Tain ordered him to do as a member of the Obsidian Order was his “step too far” that caused his break with Cardassian ideology.

Thats a wonderful thing about Garak is that I don’t think there exists “a step too far”. Rather I like to assume that even if it was a heinous request he didn’t follow it because it didn’t serve his own political motives. What was so interesting about Garak is that he clearly had a formal set of solid guiding principles. However to outsiders like us it looked chaotic and disjointed.