I don't have much advice when it comes to assessment- I'm not in the UK- but I was diagnosed in my teens and it has made a world of difference to my self-esteem.
Before my diagnosis I didn't know what was wrong with me, but I knew something had to be. I was clever, had always been told I was clever, but useless at using it for anything. I couldn't even focus on the things I wanted to do, which colloquially seems to be a key indicator of whether or not someone has ADHD. Struggling to do things you don't want to do is normal to some degree, being unable to complete things you love and are desperate to do is not.
Unfortunately I was diagnosed too late to make much difference to my school experience, and I never achieved the grades I wanted and was predicted. My mother had suspected autism and ADHD from my early years but was dismissed by the family GP who had little to no understanding of either. I fell apart in my teens- severe anxiety, self-hatred, depression, stress-related illness- and after a few years was referred to CAMHS by my school due to poor attendance. I tried medication and ultimately found it wasn't for me, but I spoke to others with ADHD online and in person and developed my own coping skills and techniques. Thankfully at university, understanding how my brain works far better following my assessment and diagnosis, I managed to make up for my poor performances at school. I haven't noticed any downsides to my diagnosis, personally.
(I'd still love a chance to go back and do secondary school again, now able to work with my ADHD rather than against it. There's so much I'd love to learn, and I feel like I missed so many opportunities)
As PPs have said, if you find a reputable assessor then there's no reason to distrust a diagnosis. The process is quite thorough, usually involving several questionnaires/interviews with different sources and then an in-person/video call assessment of symptoms. I hope you find the path going forward that's right for your DD!