I posted a another thread with a feature request that I think would be a good solution for this, at least on Mac - File Provider.
Would be amazing if Syncthing supported this. I know this functionality is not super popular with Dropbox and Google Drive users at the moment, mostly because those apps had 1st-party implementations of various features that now work differently (or no longer work at all) with File Provider. But I don’t think that necessarily means it wouldn’t be a fit for Syncthing.
Would require the folder being synced to be located in ~/Library/CloudStorage but I think for certain workflows that’s an acceptable compromise, if the File Provider selective sync functionality could be implemented on a per-folder basis in Syncthing.