I suspect it will create confusion to use the same word for a multi-dimensional meaning, especially when there is preexisting literature about the "autism spectrum" that used the single-dimensional meaning. I think it would be ideal to use another term.

It seems that the thing we're talking about is "a cluster of imperfectly correlated traits". Googling a bit, I see "syndrome" as one possibility (although it may carry the connotation of a disease, although that may be inevitable with autism). "Trait cluster"?

(If one wants to dig into the light metaphor: The thing you're looking at is a beam of light. You use a prism, and that splits the beam into its constituent frequencies. When you shine the light through a prism onto paper, the resulting image is a spectrum. But the original light beam itself is not a spectrum. Thus, the most appropriate way to use "spectrum" in connection with autism would be for someone to take a test that measures several different traits, and then the test results are the "spectrum". I'm not going to try to fight for that definition, though.)