API for SharePoint Spaces
Either I'm missing something, or SharePoint Spaces is a completely manually created environment.
So other than some of the objects/webparts supporting data from other sources (e.g. SharePoint lists/libraries) within the bounds of that webpart, there's no way to programmatically add content to a Space.
Or should we be able to build SharePoint Framework webparts that can then interact inside the Space?
We're working with a partner on geospatial data (maps, terrain, satellite photos, building geometry, building scans, services data (water pipes, electrical wires, etc) and more), which is currently visualised using a customised version of the Cesium Viewer, but it would be great to bring this data into a SharePoint Space instead, perhaps integrating/merging with data from SharePoint lists...

Thanks for the suggestion and the details on the scenario you are trying to support. You are correct that the focus of SharePoint spaces at this time is on creating 3D environments using the GUI authoring tool. We acknowledge the need for more programmatically generated content in spaces and are actively engaged in understanding these scenarios and evaluating when and how to prioritize this functionality moving forward.
Also note that using SharePoint PnP scripting and templates you can execute some automatic space creation today. This approach may be a way to accomplish some of your goals.