Please allow Pro to edit the actual marker that makes the symbol -- the polygon shape geometry that makes the symbol -- in a manner similar to ArcMap's Marker Editor.
Recognizing it's probably too much of a development effort to do this within Pro (else it would be done already) how about a quick way to export to an external editor?
Within Symbology Properties we can already "insert shape from file" [ref] from an external file like SVG or EMF (and PNG etc. for picture symbols). However this doesn't help in the situation where we want to start from and modify an existing symbol. This is the more common scenario.