Here is a Python script that will copy all the FBX files in all the subfolders and place them in a new folderhttps://github.com/stash8/FAB_FBX_Organizer/tree/main
An easy way to relink textures is to use Blender with the built-in node wrangler (ctr shift T) and it auto-attaches the materials. You still have to do it for each model but it’s better than manually doing it.
Import files into Unity
Save the file as a .blend
File > External Data > Pack Resources
File > External Data > Unpack Resources and choose the option to create new if needed
This puts the texture file with the .blend file
Import the .blend file into Unity and all material should be attached.