When following Esri's "How To: Configure Portal for ArcGIS with custom symbology" (https://support.esri.com/en/technical-article/000012539) you can create a custom symbol set group in ArcGIS Portal but it is a very time-consuming process to get it all setup.
I propose adding a new setting to the Organization Settings page that is a dropdown to select a group that has the symbols you want to show in the Map Viewer.
This idea goes along with the other ideas: Allow publishing of custom symbol sets and Expose Symbol Sets as an item visible in ArcGIS Portal
