Last I looked Synology was pretty flexible, but not without the expected extra complexity. You could add all manner of services into the box, including LDAP-related operations IIRC.
If your Synology can authenticate to an Open Directory LDAP server and Kerberos, you can go that way here. That is, the clients request access to the Synology and it then confirms access with your OS X Server via available authentication services. But most disk traffic "goes direct"; from client system to Synology disk server.
Attempting to re-export a share? I wouldn't touch that. Too complex and too slow, and probably also somewhat unstable given that non-re-exported shares can occasionally be flaky with mapping or when experiencing network glitches with a wired or wireless network.
FWIW, more complex server configurations — including getting distributed authentication going — require some knowledge to configure and then occasionally to troubleshoot. If you're just getting going with DNS or LDAP and Kerberos and related, the simpler the configuration the better, and you may still end up restarting once or twice as you gain experience. Put another way, consider hanging a big Thunderbolt storage array off the Mac Mini Server and "relegate" the Synology box for as a backup target or such, and work your way up to distributed authentication with non-Apple gear.
