My Homestuck shipping take that would get me thrown in Fandom Jail is that I think Dave/John is canon, it's just not reciprocated and is only really applicable to early Homestuck, and that's where and how it works best. I honest to god do not think it works as a reciprocal romantic relationship. This is because John is not interested in them, and this is Dave's bisexual awakening that he's too afraid to act on.
Dave would not be comfortable in a relationship with John. I personally think he needs the fantasy of it more than he wants an actual relationship with him. There's inherent weight and value in non-reciprocal relationships/feelings, too. It's important for Dave that he feels the way he does, but that does not mean it would be a good idea or constructive to act on those feelings. It's all exploratory thoughts, and those are extremely valuable to the development of one's relationship with their queerness.