Syncthing has ignore files which can be used to selectively not sync certain files or folders per device. I'm not familiar with Dropbox's smart sync, so if it's like a transparent on-demand sync then no, Syncthing doesn't have that.

Maybe it's possible to implement that as an extra app though, without even touching Syncthing. Android seems to have some concept of virtual file systems provided by apps so perhaps one could on-demand un-ignore files then block until they're available. (But yeah, that's definitely more than a quick hack.)