Update Available Volume Space

First, OS-reinstall should not harm your data. The admin password is reset (back to "password" in your case), and your IP address configuration reverts back to DHCP. But content and share settings are shouldn't be affected (though of course a backup is always recommended). A factory reset will destroy your data, but they did not recommend that.

Snapshots are built into btrfs, and really aren't related to RAID. They are more like "recycle bin on steroids". RAID protects your data from disk failures, snapshots protect you more from user error (particularly accidental deletion).

When a snapshot is first created, it takes essentially no space - it simply points to the actual file content. But as files change or are deleted, the old versions are written into the snapshot - so you can roll back to the older version. This is what the "copy on write" feature of btrfs is basically about. The more the files are changed or deleted, the bigger the snapshots become.

Whether they make sense for your backup folder depends on what you put there. If you are simply backing up copies of files, then snapshots probably do make sense -though you might adjust the settings to account for your backup schedule. If you are using a backup tool that already includes rollback features (or are writing large image files) snapshots probably don't make sense, and you should turn them off.

I am mostly backing up shared folders from another NAS. So I leave them on, but I will go in and manually clean them up if they get too large, or if I do a lot of reorganization of the source folders.