Hey there will try to answer best I can.
Albina can I just make the main muscle groups without sculpting every single mass?
Sure, if you just want some general muscle groups you can do that.
There's no requirement to build perfect anatomy if you don't think you'll get any benefit from it..
Albina suppose, the process which is called "skin blending" is the answer,
Yes, that's correct.
Wrap a copy of your skin mesh to a bake of the simulated fat.
Import the bake of the skin with the facial animation etc on it.
Make a blendshape between the two meshes and paint the weights on it to reveal the relevant areas