I found two starting solutions:
use Unison, but I found no way to install this package on a Nas Synology via DSM.
use Cloud Station Drive, but Synology only propose packages for Fedora and Ubuntu and I don't found an URL package source to install the Ubuntu package with "apt-get"
Should I dig one of these solutions or is there an other alternative solution?
You can run Unison if you copy the Unison executable to the NAS. Either put it in the default PATH or let the local Unison know where the remote binary is by setting the servercmd option. You'll need a unison executable that's compatible with the one on the NAS. If the one from Debian doesn't work (I don't know what libraries are present on Synology appliances), you may need to compile a static binary. Compiling Unison requires OCaml, and depending on which appliance you have you may need to cross-compile an ARM binary (check the knowledge base).
An Ubuntu package should work on Debian too. In fact, an Ubuntu package will work on Debian just as well as Ubuntu; what could make it not work is if the package requires a different set of library versions from what Debian offers (in which case the package would only work with a specific Ubuntu release, and not with other Ubuntu releases of a similar vintage to the Debian release).
IPKG is abandoned, you should consider installing Entware: https://github.com/Entware-ng/Entware-ng/wiki/Install-on-Synology-NAS
Then follow those steps: https://github.com/Entware-ng/Entware-ng/issues/477#issuecomment-316628740
Another option is to mount the Synology directory using webdav on Debian, and sync locally on Debian (unison only sees two local directories). But it would be much slower (as webdav has a lot of overhead for file's inode/timestamp).
Sorry to answer with just a link, but I think it's quite self explanatory. Here's how to compile a binary for the synology: https://github.com/bcpierce00/unison/issues/290
