you’ve got several options…the first is the easiest, which is make a template that has all of the materials you want and set that as the template that Rhino will use when it opens.
The second is to use Renee’s suggestion of the visLoadDirectoryVisMats command. And if you want to take that to next level, you can write a macro that runs that command automatically with a predetermined directory and then have Rhino run that macro every time it starts (Rhino Options>General>CommandLists)
The last is to simply place a macro version of visLoadDirectoryVisMats into a button. Then click on the button to load those vismats…even better, you can make specific buttons for specific directories, so you can selectively load whole folders.