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.

Synology has its own sync apps. You can also use NAS to host NextCloud, which has sync apps too.

I often mount filesystems with samba or iscsi, no syncing necessary.