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 in ArcGIS Portal but it is hidden from the users list of items and does not show up in the group that is used to populate the symbol drop down in the Map Viewer.
I propose creating a new item type for Symbol Sets in ArcGIS Portal so the symbol set is visible to users and expose details about what is in the symbol set, such as a list or grid of the symbols in the set that is visible on the item's overview tab.
This could be combined with the idea Allow publishing of custom symbol sets
