Thank you very much ! I finally managed to get the result I wanted !
I just set all my texture samples to wrap so all my Albedos, Specular, AO, Roughness maps, Normals and Metalness so my texture would still show up no matter how many samples I blend. In that way, I managed to put all my roughness map into a layer blend and plug it in into Roughness and so on for every other map. And last step I did is reset all my Landscape_LayerInfo I first created with only albedos and normals. And by resetting them with all my parameters plugged in allowed my to finally get all the infos of my material on my texture layers.
