Well, besides the sources you mentioned, I, as a solo indie dev, am a fan of “Free for the month” content. More generally, a fan of the Epic marketplace. I am collecting these content every month (everybody should). Sometimes you will find very, I mean VERY good assets to populate an environment. Also, since it is integrated with the engine, it is easy to import the stuff.
Another thing is, you can use terrain generators. Such as World Machine, World Blender, World Creator, Gaea, TerraForge3D… There are dozens of them. When you create your terrain, you can split it, and import the parts into UE as static meshes. Notice that, most of these are paid softwares.