... you seem to be confusing the term "flashback" with basic character introspection on previous events. This introspection is just basic competence in writing three-dimensional characters.
I am using "flashback" as a general term for either flashbacks as we all know them, "remember when" conversations, or character introspection where they "remember when" to themselves. A dash of that is fine... 2/3rds of the first half of the novel is excessive. More in the latter half is abusive.
My point is, I want NEW plot, not a rehash of old plot. I essentially paid for a summary of novels past, plus a short story about ancient Bajoran resistance movement, a description of DS9 mkII, and a teaser for the next novel.