As I wrote on another thread asking a similar question, yes I think it is a good idea to have a diagnosis, and the earlier the better. If the right support and the right level of support is put in from the earliest years, your child has a better chance of achieving the best possible outcomes.
It is well documented that those on the autistic spectrum are at a higher risk of mental ill health than the general population.I know a girl who was not diagnosed with ASD until she was 15, and then only after having a complete nervous breakdown. She was then in and out of education for years, and wasn't able to go back into full time education until she was 19, when she had an EHCP, and specialist provision was put in place.
Our son was not diagnosed until he was nearly 8 (although we had recognised the signs and been raising concerns since he was 3). Before that he was considered to be naughty/awkward/lazy/rude. None of these labels were remotely helpful or supportive.They just gave the teacher a reason to shout at him and keep him in at playtime, another reason for our son to hate school, and 15 years later, another issue that had to be addressed through therapy.
I have found over the years, though, that having a diagnosis has been immeasurably useful and empowering.
Because of that diagnosis I read up and researched everything I could about autism, so I knew what it meant for DS and I had a greater understanding of his needs, and was able to think of strategies to help him which made our lives at home easier.
Our son's diagnosis informed our choice of secondary school. He didn't go to the school up the road, with his brothers and all his classmates from primary school, where the SENCO showed complete ignorance of autism and didn't give a toss.That would have been totally the wrong place for him. He wouldn't have lasted 5 minutes there.
Instead, we chose to transport him to the school 6 miles away in the next county, where the SENCO was knowledgeable and compassionate, and staff had huge experience of ASD, where there were links with a special school for children with autism, and where there were facilities for learning in smaller groups and 1:1 in a quiet and calm environment. Because it was the right school, our son settled very quickly and was very happy there.
Over the years, the knowledge that DS has a diagnosis of autism has empowered me to stand up for him and explain to teachers and others who didn't understand him, about autism and how it affects him, and given me the confidence to insist on reasonable adjustments being made for him, knowing that his diagnosis means he is protected from disability discrimination by equality legislation. Even if it is only simple things like asking for him to be exempt from wearing a swimming hat because of his sensory issues. (And believe me there is still a huge amount of ignorance about autism in education circles -you only have to read some of the threads on here to see that.)
But crucially, though that diagnosis has been the passport to the fabulous specialist college where DS has been for the last two and a half years, where expectations are high, where he has made outstanding progress in all areas, where he has acquired the confidence and the skills to do things I would never have thought possible, and where he is so happy that the debilitating anxiety he once had is now hardly noticeable.
If he hadn't had a diagnosis of autism, which is part of that college's admissions criteria, DS would still be stuck at home after the disastrous experience in mainstream FE that reduced him to a complete nervous wreck, and almost broke him.
He would still be hiding away in his bedroom learning nothing, withdrawing further and further into himself, worrying incessantly about his future, and spiralling into ever increasing anxiety and depression.
I am not sure what you mean by "do you run the risk of having the breadth of your personality reductively shoehorned into a diagnosis by both yourself and others?" But if you mean does this mean expectations are lowered , then no I don't think it does.
I think it's about understanding and finding recognised strategies of managing the manifestations of autism (eg channelling and redirecting "fixations") while keeping anxiety levels low and maintaining good mental health. I think it's about having high but realistic expectations, that change over time with the right support and the right education. Indeed, our expectations of DS get higher all the time, as he continues to surprise us with his achievements.
I like to think our son would still be the same gentle, quirky, endearing young man without a diagnosis, but I know that with it, he has been enabled to do so much more than he would otherwise have done, which has given him and us, hope for the future.