A diagnosis requiring a ton of additional information, caveats, corrections, doesn't seem particularly helpful.
Maybe not, but that isn’t unique to autism and wouldn’t change even if you broke the autism diagnosis down into different groups.
What is it you think they all have in common that is useful either for funding, research, treatment, support or prognosis?
Well they all meet the diagnostic criteria - “persistent difficulties with social communication and social interaction” and “restricted and repetitive patterns of behaviours, activities or interests” that “limit and impair everyday functioning”.
Obviously prognosis, support and treatment varies depending on individual needs. But that would be the same even if you broke the diagnosis down into different groups. I’m not sure what funding you mean, but if you mean e.g. funding linked to an EHCP then it is dependent on need, not diagnosis. Even if the diagnosis was broken down researching one particular group will still have a wide range of presentations.