It was helpful to me to have a trained, experienced professional look at all of the evidence and say "Yes, you have ADHD - your symptoms/behaviour/traits are consistent with this diagnosis and not likely to be caused by another diagnosis". (They generally screen for other similar-presenting issues during the assessment process).
I had intended to go on medication, indeed she recommended it straight away. But long story short, I didn't get properly medicated until 7 years later.
It was still extremely helpful to have the diagnosis, because that confirmation by a professional enabled me to fully embrace it rather than constantly second guessing myself and wondering if I was just making excuses, this is totally normal, adult life is just hard, and I'm lazy and useless, which is basically what my mental explanation was for my own difficulties before I got the diagnosis. In the period between suspecting ADHD and getting diagnosed, I would have periods of "Why don't I try this ADHD type management tip and see if it helps?" but ultimately I'd get hit with that imposter syndrome/feel like a fraud and not invest 100% into it because of those feelings, so it would inevitably fail and then I'd spiral into depression again.
Since diagnosis I have felt like it was totally legitimate for me to buy books about ADHD, listen to podcasts about ADHD, speak to other people online who have an ADHD diagnosis, etc, and doing all of these things have been really helpful. Maybe some people find it easy to do that even without an official diagnosis - for me it was very difficult and I felt self-conscious about it too.
I should say that I got diagnosed in 2016, so the waiting list was something ridiculously short like 3 months. I don't know if I was facing the same wait today, that I would have the same feelings. For example, my 6yo is on a waiting list for Autism/ADHD diagnosis. I am pretty confident that he has either one of them possibly both. And I really don't feel as self-conscious about accessing support materials/online support spaces although I am upfront and state that he is awaiting assessment and we only suspect. This could be because I'm more confident in my judgement because I'm older and more experienced. But it might also be that I've kind of been forced into this confidence because we have been waiting so long and the issues aren't just magically going away, in some ways they are getting worse. The pattern is very clear, anyway, whereas when I first made the application for assessment, I was less sure about what they might later find.
It seems to me that with the waiting lists being so long, it would be useful to have a sort of pre-assessment which helps screen out some common-ish issues which can look like ADHD symptoms (sleep problems, vitamin deficiencies, coeliac, thyroid issues) and then if the concerns still remain for ADHD, keep the person on the waiting list and offer details of local support groups.