Sync isn't backup. Backup is for recovering data when the primary/main instance fails or gets deleted. Sync is for availability of dat, e.g. offline.
Drive does sync but also has historical versioning. If you backup the Drive application and data folders using Hyper Backup to another device then you have sync'ed data with recent changes to rollback to plus longer term backup not on the original disk.
You can sync 2 folders of the same NAS using the "TASK SCHEDULER" of DSM => see attached picture
You create a task that executes the linux Shell command here below, and define when this task is executed.
This is a pseudo realtime sync, since it runs on a periodical basis (I have a sync every 10 min)
rsync -a "/volume1/sharedfolder1/.../folder1/" "/volume1/sharedfolder2/.../folder2"
This command Synchronizes the CONTENT of folder1 with folder2
Any change made in the content of folder1 is reproduced in folder2 and vice-versa
* The "..." can be replaced by any folder path going through your folder structure, you must always start with a shared folder at the root of your volume (here "sharedfolder1")
* Be aware of "/" at the end of "/volume1/sharedfolder1/.../folder1/", and not present at the end of "/volume1/sharedfolder2/.../folder2". This indicates that you sync THE CONTENT OF folder1.