How to set up Time Machine to back up to a specific folder in my Synology NAS?
Time Machine requires a drive or shared volume. Time Machine will not work with a nested folder. On your Synology, set up the destination directory as a shared volume to your Mac.
See Synology's How do I back up files from my Mac to Synology NAS using Time Machine?

This article will guide you through the process of backing up data from your Mac to Synology NAS using Apple's Time Machine, a backup utility included with macOS 10.5 and later.