Didn't read the whole thread but
left1000 not all assets are created equally. FG is one of the most prolific authors but there are lots of better authors out there, sometimes with good assets all broken down, some with combined mesh, some not. Honestly though if you were making all this shit from scratch yourself in Unreal Engine you would need an entire team of pros and a million dollar budget.
There are tools you need to learn for Daz in this instance to make stuff your own. Try "surface selection" then 0 opacity the items you don't want, or "geometry editor" and delete nodes or even "mesh grabber" sometimes and push them out of the way. Finally, sometimes you just have to use brute force. In my game, the school needed another hallway and the model was just one big mesh with no nodes to edit so I hand painted the hallway into Photoshop. Not fun but you do what needs to be done.