I have a map in ArcGIS Pro and I set the data source of all the layers to existing services on my Portal. I'm trying to share the Web Map and it's still showing config and such to try to get me to publish new Map / Feature services. Is that a requirement? I would hope not because you might just like using ArcGIS Pro for symbology and such to make new maps rather than having to do it on the Portal map viewer that is more limited.