Thinking of Deep Space Nine as "the Dark Star Trek" without digging into what made Deep Space Nine work is so reductive. Like when people discuss Star Trek being "dark" now people bring up "DS9 was Dark and you loved that! Trekkies would hate it now!"
Deep Space Nine didn't work bc it was Dark and it wasn't Dark out of nowhere. Deep Space Nine is intimately tied to TNG in a way no other series is with another (Voyager could've been just as rooted in DS9, but. Y'know. Wasn't). Not only in characters, but that the show is so devoted to exploring deep cuts from TNG: the Bajorans, but also the Ferengi, long dismissed as failed villains, and the Trill, one-off aliens-of-the-week who DS9's writers turned into one of Trek's major species. The central thesis of DS9 isn't that the Trek Universe Is Fucked Up Actually. It's that things get more messy and complicated when Starfleet has to stick around and not dash off to another planet at the end of the episode
DS9 is darker than other Treks, yes, but DS9 is also the warmest, with the most grounded, human characters, not in spite of the fact that two-thirds of the cast are aliens but because of it. The writers treat alien characters as not representatives, but individuals. They treat everyone as individuals, with foibles and flaws, not as perfect, straitlaced future people. DS9's dark episodes are darker than other Treks, but also it's more willing to get silly and emotional. Only DS9 could do the "Sisko confesses to a conspiracy to get the Romulans in the war" episode, but also only DS9 could do the "a holographic lounge singer tries to get Odo and Kira together" episode right after it. Boiling the entire series down to "Deep Space Nine was the Dark Star Trek! Grimdark!" is...just not it