There is now a “supported” packaging of Syncthing for Synology. Instructions and package source here: https://docs.kastelo.net/synology/. I welcome testing and reports of success or otherwise, as well as requests for clarification of the documentation.
For issues with the package or it’s documentation, please open issues at the kastelo/syncthing-synology repository, or let me know in this thread.
Note that that this is not an “official” package in the sense that the Syncthing foundation or open source community is behind it. It’s a third party packaging much like any other, it just so happens that I work with/at the third party in question. It could conceivably become an official packaging at some point, but that would require a community around it first and I’m not going to foist responsibility of this thing on the Syncthing community at this point – while Kastelo currently has commercial interests in maintaining it.