I agree that storing large amount of files in a Zip or some sort archive and then unzipping would be ideal, but then you don't get the revisioning and tracking etc of the files like you do in git based project. That aside..

As per @peterduniho suggestion, to bulk change the files in visual studio, if you edit the project file (right click the project root -> "edit project file") in text, then find all your content tags and add a "CopyToOutputDirectory" child node with a value of PreserveNewest, this should change them all at once.

This can of course be done in bulk as a find/replace operation

Find "/>" Replace with "><CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>/>"

Before find replace

<Content Include="GCDS\Customisation\CUIs\Cuix ICONS\2D_to_3D.bmp"/>

After find replace

<Content Include="GCDS\Customisation\CUIs\Cuix ICONS\2D_to_3D.bmp">
  <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
</Content>