@SummerHeatwave I guess it would depend on how your daughter manages in real life - if she isn't struggling in any particular area, and most importantly she doesn't feel the need for a diagnosis, then I would leave it.
My DD has struggled with depression and an eating disorder (as well as possibly mild? sensory issues, amongst her most obvious traits) which all came to the fore and necessitated CAMHS intervention and subsequent ASD diagnosis.
My daughter was only 16 at the time of her CAMHS referral, it took about 18 months in our area to reach the top of the waiting list for an ASD assessment - in which time we had gone private because she needed the help quite desperately.
For her, the benefit was not just the diagnosis but also counselling that came with it (which we paid for privately, because of her related issues). The counselling helps her with understanding herself and I think, to not feel so alone because she wasn't able previously to understand her interactions with other people so easily.
As your daughter is already 18, she will have to be referred to the adult mental health service. In our area the waiting list is over 2 years unfortunately, and it has to be done through the GP (no self referral).
And yes, looking at family members now I think I can see autistic traits in some of them, they just seem super concentrated in our DCs!