Hi,
There are already quite a few great macros for managing custom properties available on a forum. Here is my contribution.
This is an Excel macro and it allows to read and write custom properties directly from Excel. This macro is based on Document Manager, so you do not need to utilize SW license to use it and it works lightning fast (few seconds for thousands of files).
Video Demonstration: Lightning-fast way to manage SOLIDWORKS custom properties from Excel - YouTube
SolidworksApi/macros