I'd argue there is actually a fairly simple rule of thumb when it comes to normal map vs geometry - which is that if it contributes to silhouette you should be using geometry to represent it. This is heavily context dependent - but it's a robust rule there are of course other reasons to have geometry. one thing I will add…
"Is it possible to transfer a Maya (or Blender) rig directly into Unreal using FBX and keep the weighting?" That's kindof my point : by tackling a tiny portion first and taking it all the way down to the finish line (instead of doing a full character), you'll force yourself to run into this kind of question earlier than…