My impression has been that a complex refers to a very small number of very closely related species for which the lines between species are fuzzy for some reason, either due to hybridization or difficulty in identification. Whereas species group could refer to any monophyletic group of species below subgenus (or genus in genera without subgenera). So I do think it would be useful to have an official species group rank between subgenus and complex.
I am not sure how apply it in relevant cases I can think of though (such as Condylostylus and a couple hover fly genera) because the species group may be super useful for identifying but only casually mentioned in papers, or named formally in some papers and informally in others. The same issue applies for complexes though so maybe this is a general issue more appropriate for another thread. Your case seems more clear-cut.