Not sure if I’m using the right terminology, but is it possible to sync only the stub of a file, and then sync the actual file on-demand? Similar to how OneDrive and some other consumer syncing apps work.
It does not. There was Syncthing Lite for Android that did allow download-on-demand, but it’s not maintained anymore. The feature offered by OneDrive (and Dropbox) is complicated and requires very tight integration with the OS, so I wouldn’t expect something like it implemented in Syncthing in the near future, if ever.