How can I set up NextCloud to auto backup all of my Windows Documents folders?
Nextcloud or I should say the Nextcloud Desktop Client is not a backup software. it’s primary purpose is to make the contents of your Nextcloud server available on one or multiple computers and not the other way around. Also synchronisation is not a backup, because if you delete or change a file on one end it will get deleted and changed almost instantly on the other ends too.
What you can do though, is using Nextcloud as a storage target for backups. Although it is not designed for this purpose and there are more efficient and more performant storage solutions for this purpose. But basically you can use any backup software that supports the WebDAV protocol, such as for example https://www.duplicati.com/
you can easily add more Sync Connections in your client. But as @bb77 stated - this is a sync and not a backup (but having file versions and trashbin activated on the server side it acts like a backup). additional server-side backup makes your files really safe.
Nextcloud is not a backup.
But if you backup e.g. daily your nextcloud server you can perhaps use a Nextcloud client to sync your data to Nextcloud server and from Nextcloud server to your backup server.
On error on your Nextcloud client also the data on your Nextcloud server has got an error (because of sync). Then you need the backup.