In case you're not using a model (which would need a refactoring of this section), you could ditch the file handling here, the settings don't seem to require file type inputs.

$config not known here, either push the validations to a model or fetch the config object like https://github.com/opnsense/core/blob/d5cdddedea412c54d7c9ac5c93d6680ce2fa2e21/src/opnsense/mvc/app/library/OPNsense/Backup/GDrive.php#L143-L146

maybe better to place the settings in it's own container (for example system->backup->nextcloud->..), even better would be to use a model class to handle validations.