If the boundaries between neurodevelopmental disorders lack validity – certainly they lack biological validity, and it has been argued [0] that they lack construct validity as well – maybe we should respond by merging them? Merge ADHD, ASD, etc, into one single overarching disorder? That is one way of interpreting Christopher Gillberg's ESSENCE proposal. It is also, at least somewhat, the recommendation of Lynn Waterhouse's book
Rethinking Autism: Variation and Complexity, which ends with the proposal to replace ASD with the much broader concept of "phenotypes of neurodevelopmental social impairment"
[0] See https://doi.org/10.1002/aur.1832 in particular the authors' claim that "our review of research has led us to conclude that using the ASD diagnosis in basic
and translational research cannot provide valid conclusions because the diagnosis lacks both biological and construct validity" and also "The ASD diagnosis lacks boundary construct validity..."