You use it to take the NextCloud files (set the NextCloud directory as a source) and Then you sent the files to a backup repository (destination).
You don't send the backup files to a NextCloud install. That's not the correct way to use the tool.
I self host a Borg repository in docker so the backup gets sent locally and then I also use borgbase as offsite backup.
You can have as many sources and destinations as you want
You're github mentions you tried linkwarden but still decided to build this. What features were you missing in linkwarden? It seems to do most of what you want in terms of bookmarks and archiving.
Looks good and thanks for using SSO!