Disclaimer: Haven’t actually tried this, just making an educated guess based on a few minutes of utilizing my google-fu.
You can place objects 10 kilometers away from the origin in any direction, so effectively you have a 20 kilometer world. This means that to make these levels all you have to do is just make them, none of the examples you’ve shown come even close to being 20 kilometers long.
One thing you might want to consider is rebasing the coordinate system onto the player every so often. That will ensure you don’t get any floating point errors. However, I haven’t tried manual rebasing without world composition turned off so I am not 100% certain that it works. If you test it out do report back.
As for asset loading - there really aren’t that many unique assets in a level such as that, so it’s not really a memory concern. You just hide them when they’re far away (again, I am not sure, but I think you can have a movable occlusion volume attached to your player, alternatively just do the hiding logic in your game code).