Does it matter?
I don't say this to downplay ADHD - and I think that diagnosis can be really helpful, but generally diagnosis helps when there is a problem that needs addressing.
Your post describes a happy girl who is doing really well at the moment:
"She is doing very well at school, is reading well above her age level and her teachers haven’t mentioned anything but I am a bit worried."
So there isn't really a problem to fix? All a diagnosis will do at this stage is give a label, which can be helpful, or not, depending on how it is used by those around them.
You might be fully accepting and open minded but not everyone is, and once you attach that label, you open your daughter up to their judgement/different treatment. For example, as a dyslexic kid - I was often not asked to help out with jobs involving writing/reading that I could have actually done because people had the label of "Dyslexic kid" in their mind associated with me. Labels can be hurtful if people start to lower their expectations or limit how they think of someone because of them to a point that undermines their capability. Obviously, accommodating for conditions is important, for children who are actually really struggling or don't have the capability, but it can be harmful if the interpretation of the label doesn't match the child's actual ability.
Personally, I would just bank it in the back of my mind - and if there comes a time later on when she actually has difficulties or starts to have problems in school/life, then you are already ready and aware of it. But personally, trying to get a diagnosis when there is no problem to fix doesn't seem very productive.