I do get involved at application stage as well as appeal. I do see families awarded SMI, and it’s becoming more common since as you rightly say, case law has established precedent that autism qualifies. In cases of profound disability, it’s more common as you tend to have a lot of professional evidence.
You sound very knowledgeable and articulate, and capable of advocating for your child. I imagine you filled in a very comprehensive form and attached relevant supporting evidence. Unfortunately, families in which parents themselves struggle to do that, or struggle to accept their dc’s disabilities, or become overwhelmed by how upsetting it is to have to frame everything so negatively and spell out your dc’s struggles on paper, tend to need support to secure the correct award.
It is common for autistic children to be awarded MRC and LRM without much argument, if the forms are completed properly (either the family is articulate and savvy or they have support). The higher rates are harder to get.
My dc get LRM and I think that is the correct award. I don’t feel that the SMI criteria applies to them. One gets HRC, the others MRC. The one without an autism diagnosis gets the higher award, and actually does have a higher level of need, so the award is correct. So much depends on how the forms are completed, which is unfair.
When they transition to PIP at 16, it becomes more of a battle. I wouldn’t expect you to have difficulty obtaining PIP for your ds, given what you say about his needs and that you have plenty of evidence.
I hope that answers your question. Happy to discuss further if not!