I agreed with most of your post except this part. Deep Space Nine is grittier than TNG by a wide margin, but DS9 was not the first gritty Trek series. That would be TOS. TOS's universe is downright fucking terrifying. The show deals with extremely disturbing subjects like the spread of deadly disease, PTSD, mental illness, mutually assured destruction, and many eldritch horrors lurking in the galaxy just waiting to wipe out whole swathes of civilization.
The episode with the giant space ameboa starts with a sister ship of the enterprise being destroyed, and then when they arrive at the star system in question, Kirk comments that there were billions of people on that world. Gone. Dead, just like that. And that's just one example. More than once the Enterprise comes across a civilization which has suffered some kind of apocalypse. In one case Kirk is ready to straight up annhilate an alien civilization simply because they're hostile and he feels he is out of options. DS9 barely shows us anything like that; Sisko fires toxic torpedoes at a planet with a few thousand Maquis once. Big whoop, Kirk was willing to just wipe out an entire civilization.
There's also quite a bit of people losing heir minds on the show. In some cases the acting is terrible and the mental breaks look hilarious, but in other cases its actually pretty disturbing. Highlight include Dr. Daystrom losing his shit and Commodore Decker sobbing over the loss of his crew that he feels personally responsible for killing. His character arc straight up ends with his suicide, including a scene where both Kirk and Spock try to talk him down and fail.
Don't even get me started on how dark and violent Wrath of Khan is.
DS9 is gritty Trek for sure, but TOS beat it to the punch decades earlier. Its one of the reasons its still my favorite out of all of the Trek series. I think RLM sells it a little short when they call it a "horror series" because that's not what the whole show is about, but at the same time yeah, it can do horror just fine when it wants to.