Problem when I bake my texture in Blender

Hi. In the outliner (top right corner) you have various body parts (body parts under rig, your reference, your nude body). Before baking, you need to disable rendering for them by unchecking the camera icon

The inner geometry is needed only in places where it is visible. For that reason we export the garment as thin and add inner geometry in Blender.
When doing it, we move the uvs of the created geometry away from the uvs of the outer geometry. The black artefacts you're seeing are caused by the fact that the uvs are overlapping.
If you plan on sharing your garment with other people, you must remake it to be a lot less high poly. A garment made of 124k tris will crash the game. You should not go past 10k unless necessary.
Another thing, do not use Remesh in MD. It only makes it worth. If you do not want to retopologize the mesh, at least set it to quads and choose a reasonable particle distance
By the way, use Nearet face interpolated when transferring uvs and weights

The sims 4 does not require a strong computer /graphic card to work, which means 2 things: first, it is not optimized to render high poly meshes like, for example, Second life can, second, a lot of people are playing it on worse computers than yours. I have no problem playing the game but the moment i put a 60k hairstyle in my Mods folder, the loading time noticable increases and the game crashes the moment I choose it in CAS. My pc is not the weakest among those ts4 supports.
Clone any maxis outfit and see for yourself that their polycount rarely goes beyond 10-15k tris.
If you plan on sharing your custom content, you must be considerate of people who will be using it in their game.

What you should do is go to MD, raise Particle distance, cancel Remesh, quadrangulate the mesh, export it as thin, adapt it in blender

If it's in quads by default, its good then, so long it's not remeshed.
Then, if you exported it as thin, does it mean you added thickness all over the place yourself in blender? Add it only in places where it would be visible

Are you sure you export them as thin? See where "thin" option is? Is it checked?