There doesn’t seem to be anything within the last 6 years on this topic, so of course nothing works because both Blender and Unity have jumped light years (well, at least Blender has!).
Basically, if I use the super basic human rig from Rigify, then import to Unity, everything does work fine. However, the basic rig is not what I need. It does not include fingers or a face rig. So when I switch over to the human meta-rig in Rigify, I can get everything working in Unity except the face rig. Like, literally the face bones float a mile above the body, taking the skinned verts of the face with it. Nightmare fuel. I’ve tried reparenting these face bones to DEF- bones, to ORG- bones, to MCH- bones…nothing works.