Honest question, but why do you need to confess? Is it so that you can show mutual commitment, or do you feel like they deserve to know how you feel about them?
I don’t know much about queerplatonic crushes, but do you think they know? And do you think they would be understanding? How does their allistic nature factor into your current friendship. Do you sometimes have difficulty communicating or understanding each other.
My first instinct is to talk about this in a way that makes you as comfortable as possible, so that you don't get overloaded. It can be through writing, or when you both are engaged in some activity together. Having a serious "talk" might be too stressful and some information might get lost or misunderstood if you are rushing to get everything out.
However, I don't know much about queerplatonic relationships, so it's up to you to read into the situation, hence the questions at the beginning. Be prepared that they might be looking for a romantic relationship, or just prefer being friends without the commitment.
I wish you the best!