Its cultural and scholarly centers were Timbuktu and Djenne. They were effectively inaccessible to Europeans during their golden age, but they had established connections to Morocco and Egypt. Hundreds of thousands of historical manuscripts were preserved in Timbuktu. The manuscripts are still largely unstudied, though I guess the digitization efforts should have finished by now.
With all due respect, not mentioning applying modern standards to an ancient culture, that sounds as Monthy Python-esque as it can get.
Does the Songhai empire really have nothing besides size, gold and slave trading? Surely they must have had something. What about architecture? Ancient Greeks, Romans, ancient Chinese, etc., all had impressive achievements, and they were around many centuries before the Songhai empire. Perhaps even Meso-American empires and civilizations had achievements relatively speaking, especially considering that they were much more isolated from the rest of the world than the Songhai empire.