Thank you for your post! This functionality is now supported in ArcGIS Online and is planned to be supported in the next release of ArcGIS Enterprise.
Wondering if this answers this concern: https://community.esri.com/message/739009-re-use-a-pie-chart-as-a-symbol-in-agol?commentID=739009#comment-739009
See also: https://community.esri.com/ideas/8312
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