How has a diagnosis and an assessment helped your child?

I have a 13 year old child who identifies as non-binary, assigned female at birth. They're a twin and having lots of issues at school. They are bright and intellectually competent but mostly do not engage in school work. They often refuse to be in class, or are disruptive, or unresponsive. They are receiving minimal education as a result. Lately, they are becoming less cooperative and leaving school. School are trying really hard but don't seem to know what we are dealing with. I suspect autism with a PDA profile, and the ALNCO thinks that's possible/likely, but we are  not qualified to diagnose. This behaviour started to emerge towards the end of primary school/start of puberty

At home, it's difficult to get them to comply with many demands. They are only interested in playing on their phone, watching TV or hanging out with their friends.

My kid isn't really able to articulate why they are struggling. They have said that they feel overwhelmed and like they're going to cry, so that's why they walk out of class. More recently, the response is "I just hate school" or "so and so teacher is a @&*^!"

They are being assessed for ADHD and it looks likely that will be confirmed soon. I am considering asking the Lorna Wing Centre to carry out an assessment. My husband is reluctant for our child to be labelled and have to go through a lot of invasive assessments to achieve a diagnosis with out being clear about how it will benefit. In a nutshell, why is it helpful or worthwhile.

Can anyone else help with the benefit of their own experience? Is a diagnosis necessary? I want to avoid punitive strategies. Are there any other routes to finding out strategies that would help us support our dearly loved child. It's so hard to see them having a tough time.