OK, apparently you're basing your arguments on books I've never read, since you are speaking of these unlikely things as fact, so, I have no real knowledge to debate with.
No, I don't read DS9 books (A Stitch In Time
and The 34th Rule
notwithstanding). What I say happened to Bajorans vis-á-vis them joining Starfleet is all there explicitly in TNG episodes "Ensign Ro" and "Lower Decks" and in VOY episode "Good Shepherd", as mentioned.
What I suggest is that Cardassians would behave more or less the same way, only with the names of the players changed, because they are facing more or less the same situation - occupation by a force that looks down on them and thinks it's doing them good by occupying, even though the occupied people think they have fallen from the heights of their civilization due to a barbarian invasion.
Bajorans didn't join the Cardassian military when Cardassia occupied them (as far as we know!). Don't you think it rather unlikely that Cardassians would join the UFP military when the UPF occupies them, a priori
But they still underperfomed martially in that war
Hmm... This is lightyears away from "they made a pretense of being tough guys".