Almost all the blocks are low poly.  Great. But what about not low poly? I love using this as a level designer but I need some better assets to work with that are more finely detailed. I own tens of thousands of assets but I have no idea how to get them into asset forge or if they will work even if I do???

The blocks are low-poly because the more blocks users add to a model the more polygons it will have, having high-poly blocks would raise the polygon count to such amounts that it would be unusable in games. If you want more details, consider using the greeble tool (hold G when placing a block to place it on a surface), adding (custom) textures or adding more details in a tool like Blender.