I was a sceptic about ADHD until I read Adrian Chile's articles about his adult diagnosis [1]. I was then encouraged by those close to me to go and get checked out, and sure enough I was diagnosed (at 52). Then I learnt more about it.
The shock for me was when I filled out the screening questionnaire and mentioned to the psychologist when handing it over that "this is a waste of time, everyone would answer all these questions the same way". She gently disabused me of that notion and it turns out my answers were very different to the mean. As I became aware that most other people experienced the world very differently to me, it rocked my world.
I find the label ADHD unhelpful, personally - as you seem to. It has gone from being a pejorative label for energetic kids, to being shorthand for "I can't concentrate at the moment, give me a break", to being a middle-class lever to help your child get extra time on exams. I can't fathom this one - at school I would either finish my exam in half the time allocated, or you could give me 10 hours and it wouldn't help. What good is 30 extra minutes?
Anyway, the diagnosis has been great in helping me understand myself and others. ADHD is simply the current label for a chunk of the bell curve. There is much variation within this chunk. Neurotypicals are the big bit in the middle of the curve. I am sure that over time the definitions will become more specific and nuanced.
I don't suffer from depression anymore, seems it was simply a side effect from struggling with ADHD and "why am I incompatible with normal life".
[1] https://www.theguardian.com/society/2020/sep/30/my-treatment...
(and he has written many more since).