Hi Theo,
Thanks for your questions. You are correct, we included fewer symbols in the ArcGIS Pro installation, purposely to keep the install small and fast. We are moving toward a model of delivering most of our styles through ArcGIS Online. This also lets us update styles independently of software releases.
So, to get at the styles from ArcMap, you can either import them from your ArcMap install, like you pointed out. Or, you can go to ArcGIS Online for ArcGIS Pro versions of them. In ArcGIS Pro, on the Project pane, on the Portal tab, click All Portal and search for "stylx". Right click any of those and click Add Style to add the style to your project (and populate your galleries with its symbols.)
Also, you can use any of the 3D symbols in 2D. An image of them will draw, so they draw quite efficiently. Just expand the headers in the Gallery and you will see them all.
We are always looking to update our symbol offerings. It is an active on-going effort, so we would love your input. Let us know what kind of symbols you'd like to see more of at ArcGIS Ideas.
Edie Punt