I'm not sure you can have a story where the characters experience horror but the audience doesn't without it becoming comedy. See Chuck and Dale vs. Evil and Hot Fuzz as examples. Generally the point of fiction, or at least the way you write immersive fiction, is that the audience sympathizes with at least one character. The characters in the game we'd have to sympathize with would be our soldiers. So, I don't see how you can say that horror elements aren't directed at the player when they're being torn apart by giant fish monsters. And, I'm guessing you saw the demo video, what about that isn't a horror game? Just because it doesn't have jump scares doesn't mean it isn't horror.
Please don't go completely grimdark with the game setting. And I don't mean Gimli like comedic relief either. I think there should be small pockets of joy and niceness in those bases. This can even be used to flesh out the factions more:
(Recreation Commons facility flavor text from Sid Meier's Alpha Centauri) As a bonus, the game world will appear even more nightmarish and unhospitable if you add something nice to constrast it with!
I agree, for the most part. I would like to see things that indicate that they have normal, even happy, lives. But, personally, I like my dark to be super dark. I want to see my soldiers living happy lives with their spouses and children, and then see them crying as they pin their picture up to the memorial wall when they don't come back from a mission. I'm aware that's well beyond what most people would want but, like I said, I'd prefer that over feeling like I'd removed a meaningless game piece from the board.