You’re probably going to have to deal with a lot of people who don’t respect your relationship to your child very much. You know a lot about your kid, and you’re probably going to have to deal with a lot of people who treat you like nothing you have to say matters.
You’re also probably going to have to deal with well-meaning people who say things like “you’re the expert on your kid!!!”. This sentiment can be affirming in some ways when people aren’t taking you seriously, but it can also be toxic.
Taken literally, “you’re the expert on your kid” isn’t true — and it doesn’t need to be. Even aside from disability, kids are complicated. No parent understands everything about their kid. Every parent faces confusing situations, and every parent makes mistakes. Parenting kids with disabilities tends to mean being confused more of the time. That’s ok. You don’t need to be a perfect expert on your kid. It’s both impossible and unnecessary.
There will be times when you have absolutely no idea. When your kid is struggling and you don’t know why, and strategies you’re trying aren’t working. When that happens, you’re still your kid’s parent, and the relationship still matters. You’re not going to be an expert on every aspect of your kid at all times, and that’s ok.
Sometimes when you don’t know what to do, others have useful ideas. It’s worth being aware that good strategies tend to get developed in silos. If you’re only looking in one context, it’s worth trying more. For instance, there are things medical/therapy professionals often know, things adult activists living with the same disability often know, things teaches often know, and so on. It can also be worth looking outside of your child’s disability group — resources intended for one disability are often helpful for another, and groups don’t always talk to each other.
(This goes double if your child is autistic. Nothing disabling about autism is completely unique to autism; all of it’s shared with some other disabilities. Resources associated with other conditions are often better (and less behaviorist.).
All that said — you will probably face situations in which none of that helps. Sometimes you’ll seek out all kinds of perspectives and still find that nothing you’re aware of helps enough. When that happens, you may attract people who give you a lot of bad advice loudly. When you’re worried, it can be hard not to believe people who yell at you and tell you that they are experts.
Don’t get psyched out by professionals who try to convince you to stop thinking for yourself. They’re good at sounding right in intimidating ways. They often do not actually know what they are talking about. And ultimately, you are your kid’s parent, and all parents are clueless sometimes, all parents make mistakes, and you and your child are allowed to be human.
Similarly, as your child grows up, they will grow apart from you in some ways. That’s how kids are, and that’s part of how maturity works. Teenagers do things that their parents don’t understand. All the more so, adults do things that their parents don’t understand. Even in childhood, no one can really be a complete expert on another human being. Disability doesn’t change that. It’s not going to be possible to be an expert on your kid, and that’s ok. They’re a person, and so are you.
Short version: “You are the expert on your kid” is too much pressure. There’s a grain of truth, but it doesn’t reflect reality — and it doesn’t need to. There are a lot of unsolved problems in disability support — and in any case, no human being can really be an expert on someone else.