The thing about postcanon Dirk/Rose is that Ult Dirk needs Rose way, way more than Rose needs him. Dirk is trying desperately to convince himself, and us, and Rose, that Rose is in the same position he is. But she isn't, and I think on some level Dirk knows it.
This line really struck me in my Epilogues reread:
Kanaya loves Rose, but sometimes love just isnāt enough. Sometimes what you need is understanding.
Because, the thing is, this line isn't really about Kanaya and Rose. It's about Jake and Dirk. And it becomes obvious when compared with another line from later on where Dirk talks about his relationship with Rose:
Weāre family. We belong together. And after years of micromanaging the inconsistent and confused desires of total imbeciles, wouldnāt it be a relief to have someone by my side who understood me?
So here's what I think is going on. Dirk feels desperately, desperately alone. His ascension to Ultimate Self-hood is making him aware of all sorts of dire shit. He's becoming aware of his status as a fictional character and the tenuous position of the "canon" he's from. He's "drowning in his own dismal persona", as he once put it in canon. And no one fucking gets it, least of all the man he loves.
Because it's clear that Jake could never, ever understand what Dirk is going through. He does not understand Dirk. Will not understand Dirk. Willfully chooses to avoid understanding Dirk at every turn. And Dirk is so, so tired of it.
And then Dirk starts to get to know Rose better, and⦠they just click. They truly are kindred spirits in many ways. They have similar senses of humor, similar interests, similar ways of viewing the world. And most importantly, Rose is ascending too. She will soon be in a position to understand what's going on at the same level Dirk does. She'll be able to understand him. And Dirk needs understanding so desperately.
But there's a problem, and that problem is that, while Dirk feels that he doesn't have anyone else who can understand him, the same isn't true for Rose. Because Kanaya does understand her. Maybe not completely. Maybe she'll never fully be able to grasp the metaphysical implications of Rose's ascension. But, importantly, she tries to understand, in a way Jake would never do for Dirk.
And so Dirk knows that, in order for his plan to work, in order to convince Rose to come with him, he has to prevent Kanaya from getting to her at all costs. Because no matter how much he tries to tell himself that Rose would feel the same way he does even without his influence, he knows that if Rose had to choose between him and Kanaya, she would choose Kanaya in a heartbeat.
So he prevents Kanaya from reaching Rose, manipulates Kanaya into telling Rose that she should leave her and go with Dirk. And he suborns Rose's will into his own. He does everything he can to make her part of him. Because in the end, the only way Dirk Strider can understand to not be alone is to gaze at himself in the mirror.
I'll leave you with a quote from Candy:
ROSE: Thereās no one who understands me like you, Kanaya. No one who looks at me the way you do. No one who brings me back down to earth with ceaseless, brutal honestly like you.
ROSE: I have no idea whatād Iād do without you.
KANAYA: You Would Probably Do Just Fine
ROSE: Yes, but Iād be very unhappy.