I am looking for extended or advanced symbology symbol sets

[RickBrenner]

I would like a larger set of symbols for ArcGIS symbology.  Are there ad-ins available for downloading and using in ArcGIS Pro?

[KoryKramer]

There is guidance here about adding styles to an ArcGIS Pro project: Add styles to a project—ArcGIS Pro | Documentation 

(You might not be but) if you were previously an ArcMap user, there is a section in there that is specific to importing ArcMap styles.

A quick tip for the future - when asking something specific about ArcGIS Pro, there are a lot of very helpful users with eyes on the ArcGIS Pro‌ space, so posting there could help get a response.

I hope this helps!

[BryndaHatch]

As @KoryKramer said, you can add styles to your project in ArcGIS Pro.  This is a gallery of your options: https://esri-styles.maps.arcgis.com/home/index.html