Compendium Folders

Compendium Folders adds a nested folder structure to organize your compendiums, and to organize contents of a compendium. This allows you to hide compendiums completely, in addition to being able to export folder structures into a compendium for later use.

https://foundryvtt.com/packages/compendium-folders

I have run multiple simultaneous campaigns from level 1 thru 20... to do that you have to be organized. This module allows you to create a reasonably deep folder structure and manage your content. I am not really sure why this is not part of the core Foundry VTT yet, but it should be. I have not changed the default configuration. Only once have aI had to use the Fix Compendium feature and it worked great.