What do you use for keeping files synced between different machines like Dropbox/OneDrive/GDrive does?
I often mount filesystems with samba or iscsi, no syncing necessary.
Synology has its own sync apps. You can also use NAS to host NextCloud, which has sync apps too.
If you want dropbox-like experience, NextCloud can mount external storage (WebDAV, SMB, SFTP, S3, or even another NextCloud instance). Just mount your NAS there and install NextCloud apps on your devices to access them. Some NAS even allow you to deploy NextCloud directly on that machine.