Well, real quick before I finish this week's chapter for Baldur's Gate.
I don't know the gnitty gritty of the various techs, but I do know that the UNSC has a rediculous resource base in comparison
I've addressed this, but while the UNSC has a vast and wide-spread industrial and resource base, it's not without its vulnerabilities. The major flaw in the Covenant's strategy (or lack thereof) was that they blindly stumbled from one planet or another, looking mostly for Forerunner artifacts primarily and exterminating the humans as they went along. There was very little grand strategy involved. Impart that was due to the religious zealotry in regards to their genocide and impart because the Covenant was so far above the UNSC's tech level that they didn't really need to.
and that the Cardassians will probably not play some keep away"glass from orbit" game(occupation of Bajor)(cross breeding).
Well, no. That is true. But that's because even the fascist and racist Cardassian government would be appalled at the level of genocidal mania that exists within the Covenant. What the Cardassians call a radical the Covenant calls the standard. That being said, the Cardassians are not above orbital strikes on strategic targets. A Cardassian Galor's main phaser bank has an output of ~700 MWs and even with the issue of atmospheric bloom, that's more than enough firepower to destroy any target. Anything that can resist that can be taken out by a photon torpedo strike, if need be.
The UNSC just needs one ship to reverse engineer, even partially before they can start hitting back/capturing more tech to reverse engineer then its GG.
That seems very simple.
I can assure you it is not.
Keep in mind that the UNSC had access to Forerunner technology, which was very open in integrating with human users. Keep in mind that the Covenant based the entirety of their technology on Forerunner technology. Keep in mind that therefore, the UNSC had a window and door into not only the architecture of the very programming that the Covenant used, but access to limited physical hardware. Keep in mind the Covenant were notoriously bad at cyber warfare. (in which the UNSC excelled)
Keep in mind the war lasted ~27 years. And that span of time, the UNSC was only JUST beginning to field heavy laser cannons for ground troops. Had just begun to roll out ships that had shielding technology. Had after 27 years, have finally integrated greater FTL and sensor capability into their most recent starships. After 27 years, had yet to integrate a laser defense grid system onto their capital ships. Had yet to produce plasma torpedoes or particle weapons for their ships. In fact, they doubled down upon the poorly designed MAC design and stuck to missiles and ballistic weaponry.
Now I don't say that to downplay the UNSC. The UNSC did very well, especially when you consider the amount of economic, demographic, and political damage they took from having entire worlds exterminated. And keep in mind that the UNSC saw a great deal of utility in producing weapons and designs that favored cyber warfare and asymmetric warfare. But also consider that within the span of ~30 years, the UNSC was only JUST integrating some of the greater technological advantages into their capital ships. And a great deal of this was bare minimum stuff.
So when you pitch the idea that the Cardassians losing a ship to them (which is unlikely--so remote as to be laughable) and that they will somehow reverse engineer this within a timeframe that is relevant to this war, I'm going to need to see that this is possible. Yes, the UNSC is very adaptable. Yes they have some of the best General AI in sci-fi. Yes they are great at stealing technology. No, they won't start rolling off UNSC warships with Cardassian energy weapons within 6-24 months of having obtained the technology.
The first issue is one of programming. The Cardassians would have created their own programming architecture and incorporated that of neighbor aliens throughout their growth. That would undoubtedly include some Federation bits as well, but overall, the UNSC is not going to have the easy door to Cardassian programming science that they did with the UNSC. Second, while the Cardassians are by no means excellent in the field of cyber security, they are not horribly oblivious to its existence.
Being a fascist people, security is a top priority and that is seen in the show. You generally need isoliniar rods to access high security settings in Cardassian society and enclosed security settings such as a station or a ship, where there is a fear that the station or ship might be taken--has downright draconian measures of dealing with insurgents. From releasing toxic gas to generating security force fields to using replicators to produce phaser turrets to a self-destruct sequence that requires the commanding officer to disable it.
Even IF the UNSC were able to obtain enough Cardassian hardware and software to begin an aggressive technological leap forward that they did with the Covenant's technology, the best you can hope for is ships 30 years later having managed only about half of the technologies and only in certain sectors.
The reason is one of great difficulty. Because you not only need to experiment to figure out how something works (which can take months to years), but you also need to find a way to integrate it with current existing technologies. (that will also take months to years to accomplish). You then need to move it from a working prototype to a practical field version that can reliably be deployed by your military within the expected parameters. (again, months to years) You then need to retool your industrial base to be able to produce those new technological goods on a large scale. (that will take years) Assuming you have the specific resources needed to produce it. (which if you don't, could months to years to establish said resource industries). You then need to integrate it with older ships (months to years) as well as integrate it into a brand new ship class that is better able to take advantage of that technology and other emerging technologies. (decades)
The technological disparity is so great between the UNSC and the Cardassians, that the UNSC would need decades to produce ships that could match a single Galor ship at 4 to 1 odds in the Cardassian's favor. The refinement that follows will probably take another 10-20 years. Now, the gap in technology would have considerably closed, that is the way it goes (just ask China), but by that same time, the Cardassians will have also of advanced their own technology. Cardassian technology in 30-50 years will look closer to Starfleet technology of the post Dominion War era. At which point, the UNSC (if they're lucky) will be producing ships that might begin to approach the competitive level that the Cardassians initially started with in only a handful of sectors that the UNSC can compete in.
And in that same time, the Obsidian Order will be plundering the UNSC of their secrets too. From super soldiers to power armor to their advanced General AI. And the Obsidian Order will have a greater time in catching up, because the gap between where the UNSC leads the Union is much smaller than the reverse.
I think the Cardassians could win several encounters early on and even part way through, but I just don't see them taking the encounters seriously the same way the UNSC would see it as a possible extinction event.
See, I think without realizing it, you're trying to have it both ways. You can't insist that the Cardassians will refrain from the widespread carnage that the Covenant will inflict--then also insist that the UNSC will unify as if they did. The UNSC will certainly be able to play the "invaders from outer space" card to great effect, but the fact is that the UNSC is not without its political weaknesses and the Cardassians will eagerly exploit that weakness. A weakness that the Covenant never really even considered. (because it hated all political factions equally)
The Union also won't just stumble from planet to planet; the UNSC has a large web of supply chains that links agricultural and industrial and capital worlds together to form a unified interstellar market. The Cardassians do something similar with their own empire--and so they'll know what to target. If they want to cause food shortages in the UNSC, they hit the agricultural worlds. If they want to break the UNSC's industrial back, they go after the shipyards and the industrial worlds that feed them. If they want to break the political will of the UNSC, they'll work to pit the competing political factions against each other. Specifically the outer vs inner colony.
The Cardassians can supply the UNSC's outer colony rebels with third rate technology a century old and STILL produce a highly competitive terrorist force that hits well above its own weight class. The UNSC won't just be fighting Cardassians, they'll also be facing internal pressures as outer colonies are encouraged to think of themselves as independent and not at war with the Cardassians.
It doesn't have to work so well as to break up the UNSC. All it needs to do is slow down its political, economical, and military process for it to be a success.
They would rally,command and micro everything as best they could for the inevitable opportunity that opens up.
Again, you can't just look at the UNSC as you saw them in the midst of the UNSC-Covenant War, but rather how they would react to a different sort of overwhelming threat. It's really easy to find nationalism when your option is choosing it over being vaporized by a merciless alien race that seeks to wipe out all of mankind. It's NOT so easy when your opponent is simply intent on ruling you--even in a fascist and brutal way. Because at least then you're given the option of living and the hope that it might not be so bad.
The Covenant's own internal political and religious outlook meant that it gave up all political leverage against the UNSC, while they just mostly wandered around aimlessly in UNSC space committing mass genocide. The UNSC was given massive time to ramp up their production capabilities in every sector that mattered and all the political capital that could possibly have been generated.
My impression is that the Cardassians will spent about 2-8 years conquering half of the UNSC (with Earth being a massive prize) and spent maybe 30-70 years occupying it before losing half of those gained territories, due to their own political issues back home. That would allow the remaining UNSC territories to absorb some of the technological wonders of the Cardassians and produce a fleet capable of standing up to (but not really matching) the Cardassians. At the same time, the UNSC will have funneled that technology and other special forces into undermining Cardassian rule within their new occupied territories until the Cardassian public is so sick and tired of it that Central Command is forced to relinquish the territory that they cannot fully hold.
Unfortunately, by that time--complications will have occurred. You will have a significant amount of half-breeds within that former territory that will not be accepted into traditional Cardassian family structure while at the same time not being trusted by their human side. And such groups will in the future, be ruthlessly exploited by the Cardassians so that in times of strength, the Cardassians will be able to stretch out and retake many of those former worlds. (though probably not Earth again) The UNSC may continue to exist, but will likely have formed (at least) two or three different rival power centers within its government and may even split into different governments. After decades for those political, economic, military, and social differences to set in and mutate on their own--getting along is going to be difficult, let alone re-integration into a unified government that was known for its oppression.
Overall, expect what was once the UNSC to be fractured into mini-states that only work together in mutual defense against the Cardassians, but are otherwise competitors. In such cases, historically those states will be minor powers compared to the much stronger and more advanced Cardassian Union.
That's my take.