Here's my response to the best of my ability-
One way I get around trying to "stick to canon" is realizing that the "canon" itself is deeply flawed, so I try to write William in a way that feels accurate to me. It saves me the trouble of trying to portray him in any way that limits the flow of creativity. Otherwise, I spend hours combing over details in the games. It's not fun.
When I originally encountered William, we spent a lot of time coming up with our own lore after watching several timeline and analysis videos. It didn't help that at the time this was sort of my escape from a living situation I was in IRL. So I leaned in heavily for his support. Working on a design for him that was uniquely "mine" and fashioning that imagery into something I could focus on as a source of comfort. Imagining being with him as an individual separate from canon.
In the end I settled on creating my own AU, where William could be where he felt most comfortable, while also rewriting the canon to facilitate this. In place of over-used tropes and bad writing, I put a lot of thought into fixing anachronisms with real world inspiration, such as realistic animatronics, Henry's role and his relationship with William, Fredbear's, William's motive for why he did what he did and how, and of course how he eventually dies (and comes back). Details I'm still working on to this day.
As for self-shipping, I find comfort in him as a person, rather than the character or role he plays. I relate to things about him I see in other people's portrayals. It's like we're a collective mind stemming from the same source, and it's interesting to see what comes from that. And I mean, the fanart and fanfiction really carry this fandom.
With my William, I feel this sort of paternalness to his character. He's someone who masks his true feelings behind acts of kindness and devotion. Playing a role. Acting like someone he's not. Always right. Always getting what he wants. He's obsessive, and if he doesn't get what he wants, he acts up. He's dramatic. He craves attention. He loves himself more than anyone else. It's a basic fact. And the way we explore this together allows me to self-improve with his support. That kindness he uses to get what he wants, is to see me succeed and to achieve my goals. And as long as he gets to be by my side, he's happy.
Then of course there's intimacy, among other things I won't get into, but I've learned much about myself through him. He has sufficiently altered my perspective to feel deserving of kindness, allowing for healing and growth, where before I didn't feel I was capable. Helping prove things wrong about myself I never thought could change.
He really taught me to love myself. Not just to embrace the pain, but to become one with it, and allow it to fundamentally alter you.
Part of what I do as a creator is relate to characters. I try to see myself as them, becoming them, not just imagining myself with them. I really try to get into their head. It's quite a unique experience.
So I guess my answer to your question is: Transformative media.
We create something that not only touches and transforms us, but transforms others as well. Be it opening their mind to new ideas or introducing unique spins on existing ones to the world.
Fighting getting hung up on "accuracy" is just part of that challenge methinks.