Yes, mites are arachnids, but this point is pedantic. Miticides don't work by counting legs and body segments.

Consider Amitraz [1]. It is both an insecticide and a miticide, and the method of action is on the central nervous system. Fluvalinate [2], another common one, is also broadly toxic to many different organisms.

[1] https://en.wikipedia.org/wiki/Amitraz

[2] https://pubchem.ncbi.nlm.nih.gov/compound/Fluvalinate