With respect, you are likely autistic if you've already considered you are. Describing yourself as gifted is also a clue, as well as your passion for deciphering and deep-diving. Neurotypical people rarely consider themselves to be autistic, much less make effort to find out if they are.

I have strong imposter syndrome about being AuDHD despite having an official diagnosis (but self-diagnosis is also valid). I was diagnosed late in life (at 42).

Why do you think you're not autistic? I mean, what evidence suggests that you are neurotypical? Is there any conflicting evidence to suggest you are neurodiverse? If so, what is this?