I take your points here very seriously, but I think there a couple of flaws here you’re not considering. It’s possible you (or I!) would come to same conclusions even considering these points, but I wanted to lay them out for further consideration.

1. These are not independent states like CA, AZ, NM, and TX are. They are colonies. Part of the deal with colonies is that they are not fully part of the political universe of the state sponsoring them. So a more accurate analogy would be a situation where the US ceded Guam, the Virgin Islands, and Puerto Rico to Mexico following the war. Does that make it better? Perhaps not, but I wanted to fine tune the analogy so that it doesn’t mislead us. (Also, I feel like it’s important to point out that the four states you mention contain nearly 1/4 of the United States population. If these colonies contained 1/4 of the Federation population…well, everything about that political situation would have been incredibly different.)
2. You don’t fully account for the benefit this peace treaty had, only its downsides. By all accounts the Cardassian–Federation War was a pretty violent one. We’ve met several people on both sides that lost much of their family to the war. Ending that war and ensuring peace and stability at the border was a huge humanitarian accomplishment, but, like the end of all conflicts, this one required sacrifices to be made. The Cardassians were not going to just agree to give the Federation everything they wanted. I don’t mean this to sound heartless; why did these people have to sacrifice and not the comfy higher-ups at Starfleet Command is a legitimate question. But it’s a legitimate question with a very clear answer: the comfy higher-ups at Starfleet Command weren’t living in territories claimed by Cardassians. Should the Federation have continued to wage a deadly war when peace was an available option because some people somewhere would be made slightly less well-off by that peace?
3. By what right did the Dorvan V colonists even claim ownership over the planet anyway? In “Journey’s End,” it’s made clear that a) these colonists had only been there for 20 years and b) they were warned before they settled there about the politically unstable nature of the region and the possibility they would not be able to stay. They voluntarily chose to accept that risk by choosing this planet over all other options. That doesn’t make it suck any less for them; but their entire claim to the planet was based on that assumption of risk. If there was a hypothetical piece of land disputed between Mexico and the US, and I decided to move there, and everyone said “ok but things are tense with Mexico, there’s a chance you won’t be able to stay there forever” then had to leave after 20 years…yeah it sucks because I’ve established roots there, but they were roots I chose to place in a shaky foundation.
4. Finally, would your hypothetical ceding of land to Mexico change if every person displaced from CA/AZ/NM/TX by the peace treaty were made fully materially whole again? Like yeah, sucks we can’t go to the Grand Canyon anymore, but there was every indication that the people displaced from the worlds being ceded to the Cardassians would have had the same quality of life (with respect to material possessions) that they had on their planet and that presumably nearly every member of the Federation enjoys (except those who voluntarily pick different lifestyles). The idea that we could make every citizen of CA/AZ/NM/TX while in a different location is so far outside of our capabilities as a society that it’s difficult to imagine, but the Federation certainly does. Again, the colonists on Dorvan V had a spiritual connection to the land that cannot be made whole elsewhere, I get that and I very much respect it. (Even if it’s only a 20-year-old spiritual connection.) But that is the entirety of their loss, which is another way I think the analogy you’re drawing is incomplete and could potentially lead us down the wrong path.