Regarding textures … a have my textures in a folder outside the assets directory. Creating a material from Node Wizard does not copy texture files in any way, it just creates a .blend which relatively references to this files (no absolute path). If the relative path stays constant, no problems should arise, maybe place textures parallel to the asset root folder.
The export exports all selected objects, imho … during import, all were placed in a single collection. Can you create a small .blend file and a little explanation that shows me exactly what went wrong and what you expect?