Thanks for your reply, I already wondered a few times, if it would be worth it to use metahumans. I already hit some walls, as you mentioned, but so far I was able to overcome these. I already have morphing capabilities for the body (to fat and skinny body) that work with all the clothes that come with the MH creator + the ones from the citySample. Maybe I will give up but I want to give the faces at least a few more tries, I guess 
But I’m still curious, if someone knows an answer to my original question: will it be a problem to have, lets say 20 morph targets in the face skeletal mesh to change the shape of the face (each of those affecting all the vertices) + the morph targets for animation that the MH already have. And would my alternative I posted in the initial post be a good idea?