From what you have said it sounds like it should be working. Once I import an obj file I will assign the diffuse texture for it in the same way you have said.. I make sure the textures reside in an appropriate Daz file directory, in runtime/textures folder. I then save as a scene asset file in Daz.. making sure I select to save it in the MyLibrary folder - so the data is stored here. I then save as a prop like you said, or if there are multiple parts to it save as a scene subset (but I usually change the directory to go in the props folder). Only because I mostly make props.

make sure your textures are jpeg and are square (1080 x 1080 for example) - you probably already know this.

If you do it like this you should see these files in the following locations...

MyDaz3DLibrary/data/yourVendorName/product/terrain_01 (example)

MyDaz3DLibrary/props/YourVendorName/Terrain_01/Terrain_01.duf (and png also here).

MyDaz3DLirary/Runtime/Textures/YourVendorName/Terrain_01/Terrain_01.jpg

If you plan to share your work these 3 folders and subfolders are copied...

data, props and runtime... with only the relevent sub folders present.

It took me a bit to figure it out also - I also got great help in Daz forums. They could explain it maybe clearer than me.  it is actually pretty easy to do.

Good Luck :)