Okay, it's as I thought. The polycount is way too high and the topology is terrible and horrible. MD makes beautiful UV islands but horrid topology. Always model in quads and keep your particle count as high as you can get away with. If you have body parts that clip through that you know you'll delete later don't worry about that.
I'm going to borrow Mauvemorn's screenshot.
Change the settings in MD and re-simulate your mesh.
Try to avoid using the bottom right of the
CAS map. Many creators put things there and that can cause conflicts. You don't need to resize your islands; you can move some of your islands closer together and fit everything in the upper area.
Your weight paint is extremely choppy looking and that's because of the topology. Export the mesh of whatever you cloned and look at it in Blender. You can see how even the lines are. Your mesh is made up of random triangles. Again, MD does this so model in quads.
You don't have a proper uv_1 so your mesh won't respond well to the CAS sliders. Append whatever you cloned into Blender (file > append > navigate to your file > object > select the meshes (they'll be named s4studio_mesh_something) > append from library. Hold shift to select all of the meshes if there is more than one. Delete the bone_bone_shape. If an extra rig gets appended delete that, too. Shift-select all of the reference's mesh groups if there are more than one and join them with Ctrl + J. Select your mesh, add a data transfer modifier. Follow what's in the screenshot below.
Hold shift and select the reference mesh FIRST then your mesh, enter weight paint mode. Follow what's in the screenshot below.
Delete the reference mesh when you are done.
I have a tutorial in my signature that may help, and if Mauvemorn has instructions for you be sure to follow them.
