Some context for your search might be helpful to have. Is this connected to the survival of spirituality across the Atlantic? The Angolan context, or Ndongo specifically? Missionary activity or accounts of the Manikongo? Something else?

In print, you may want to look at anything dealing with Ndongo or Anna Nzinga, its most famous ruler. The works of John Thornton or Linda Heywood may be helpful. I'm not sure this information is broken out anywhere in English--if you've been through the ethnographic journals already, I assume--but you may find some pointers in their work on west central Africa and possibly in some of the travel literature that may exist in translation. Thornton in particular has an essay specifically on Kongolese and Mbundu religious beliefs (16th-18th centuries) in Heywood's edited collection from 2002, and the two together have a more recent work on the entire interaction from 2007. That may at least give you somewhere to start, but if you don't have Portuguese I suspect it will be a difficult slog. But maybe someone who works more actively on Angola will chime in with something from anthro or religious studies that is useful, too.