> Despite that they don't get as much attention in contemporary discussion (at least, in the US), were European powers' continental African genocides morally worse, or not morally worse, than the New World's (institution of) slavery

That's like comparing Ebola to the plague, both were terrible. If presses, I'd say that European colonisation was slightly less bad from a moral standpoint, because there was usually at least some improvements in infrastructure, schools, there were some efforts around medicine, vaccination, healing, etc. and oftentimes traditional power structures were kept, just under the European colonial hierarchy. And colonial subjects usually had at least some limited rights ( mostly after the initial very bloody years/decades with genocides and atrocities which resulted in a backlash so things were scaled down). Compared to New World slavery where slaves had no rights, and were initially uprooted from where they lived to be shipped overseas for labour. And their descendants were subjected to the equally terrible "born into slavery" practice.