The cardassians (btw, that word is never capitalized, unless you are writing it as the name of the nation, rather than the people) actually have a vested interest in destabilizing the STO, both because of proximity, and the Dawiar situation.
Uh... you then immediately went on to refer to the name of the nation, and not the people.
Because if you're drawing a distinction between the Cardassian
species and the polity we know as the Cardassian
Union...
Well, the Union has an interest in destabilizing the STO. The Cardassian species does not, except insofar as the Union truly serves the best interests of the Cardassian species, which is a doubtful and self-serving claim on their part.
Also on the capitalization point, most species in Star Trek (there are exceptions) have a name we derive from their homeworld. Following the capitalization conventions common in standard English, that makes the name of the species a proper noun and the corresponding adjective a proper adjective- therefore, capitalized. We talk about the Italians from Italy and their characteristic Italian cuisine, we do
not talk about how all the italians in Italy speak the italian language.
(note capitalization in last sentence of above paragraph)
By similar logic (heh), it would be entirely reasonable to refer to Vulcans from Vulcan and their Vulcan music. Or, to keep things consistent, Earthlings from Earth and their colorful Earthling expressions. You can make an argument that species names should be kept lower-case, but that invites the question of whether Star Trek treats species affiliation more like the way we treat
species identity today, or more like the way we treat
national cultural identity today.
I would argue more for the latter, in a lot of ways. Different species are viewed as different, but not in ways that fundamentally alter their rights and responsibilities. And when a species/culture with disagreeable customs is found, the arguments for showing tolerance or interest in their customs is usually rooted more in cultural terms than in biological ones.