I do like this analysis of their relationships, and I can somewhat agree that their relationships in the comics and epilogue are kind of handled poorly, and we just don't see a whole lot of them interacting or really working through all of the problems they have. I think, however, that this is largely because Homestuck was critiquing the way fandom prioritizes ships between men rather than women, since Homestuck really fleshed out the sapphic and lesbian relationships in its story. Like, a lot of what happens between Dave and Karkat in the epilogue is just one big meta commentary on how a lot of fans don't get their happy gay ship in the end because "a woman is getting in the way of it" lol which I have always hated, I can say many things about the whole DaveJadeKat situation and how it frames the ship DaveJade in a terrible light, how even when DaveJade shippers "win" there is nothing to see and they don't expand upon it at all and there's the whole Yiffy thing (it's a lot okay)

And I think there's something to be said about how Karkat is almost thrown completely to the wayside as an afterthought because he doesn't exactly fit the stereotypical mold for these kinds of Gay ships, he's literally not human first off, and his sexuality issues are so different from Dave's that it's hard to really "place" him anywhere. I guess if you haven't read a lot of Yaoi or old gay japanese manga that it isn't readily obvious that they by and large struggle to see who is the "top" during this whole dumb DaveJadeKat arc(this is a silly distinction but it matters to Fujoshi's a lot so bear with me </3) and because of just how different Karkat is from protagonists or characters in those kinds of stories, Jade trying to get them all to be in a relationship together just doesn't really work or mesh well because the narrative keeps trying to frame it in an even more archaic and human way. Even she kind of struggles with placing Karkat "in the ship", saying they have some kind of black romance but Karkat has never actually felt that way about her (unless I am forgetting I do know they definitely fought a lot and there was the whole "if you hate yourself so much why don't you just hate-marry yourself" thing from the comics, but I never read that as Karkat has a hate-crush on her idk I would have to reread that part)

Needless to say, I think the reason Karkat is kind of treated in this offhand way as just the secondary part to this gay ship is also commentary about how a lot of men in those yaoi stories are incredibly shallow and aren't really expanded upon as deeply as the "top" is. Their feelings about whatever relationship they get into doesn't matter, their sexuality is treated as a plot device and is never "confirmed" in a satisfying way even when they're with men, and even the way the story itself struggles to define "what they are" to each other is a common trope. I have always sort of disliked the scene in Meat where Dave and Karkat finally kiss because it felt too reminiscent of those shallowly written BL (boys love) stories where the "bottom" just gets together with their one true love or whatever even though we as the reader know absolutely nothing about them as a person and really they're just seen as a thing to be 'won' or because the author just thought it'd be cute to see them together lol. Even Dirk kind of places Karkat in this role while he is narrating the whole thing, which is why it is so "icky" to me. I feel like they'll lowkey break up in Meat because of this as well, but idk </3

I can't speak a lot on DirkJake because I am not a DirkJake super fan or whatever, Idk much about them to say anything so I'm sure someone else will come along.But I definitely agree with your consensus on DaveKat and it's why I tend to write a lot of fics about them just talking about their relationship because I just wish the story expanded on that end, but on the other hand I kind of get why they don't do that since so much of the fandom cared way more for this ship between men than any of the actual confirmed gay relationships between women.