Not sure what your budget it. TRAINWORX makes a couple of great looking building flats that fit your required space. One being the Rock Island freight house featuring a great looking smoke stack feature. Others make them also or you can simply kitbash kits. These fit your depth requirements of 5 inches. I would put these up against your yard tracks. I would then use either Korber or OGR on the other side and make them follow the exact same lines as the Trainworx ones. Seeing it’s on the mainline side. I would keep them simple and make them look like a warehouse or perhaps the backside of apartments complete with fire escapes.
I think I’d go a little taller than your 6 inches if doing buildings if you go this route.
