If Windows backs up on the same day every week you could add a user script to unRAID that runs later or the next day.
I just wrote one similar for another device.
You may have to tweak it slightly for unraid/your use case. Carefully test it because it calls rm and you don't want to accidentally wipe your array (or at least comment out the rm line until you're sure it works.)
#!/bin/bash
#
# Backup critical files
#
## Options
# Number of backups to keep
NUM_TO_KEEP=6
# directory to be backed up
BACKUP_SOURCE="/mnt/hdd/share/unraid"
# directory to store backups
BACKUP_DEST="/mnt/hdd/share/backup"
# Begin backup
dest_file="${BACKUP_DEST}/unraid-$(date '+%Y-%m-%d')"
echo "Archiving critical files to: ${dest_file}.tar.gz"
tar -czf ${dest_file}.tar.gz ${BACKUP_SOURCE}
if [[ $? != 0 ]]; then
# Alert
echo "Critical Backup Archiving FAILED"
exit 1
fi
# make it readable by all
chmod a+r ${dest_file}.tar.gz
# Clear out all but X most recent
(cd ${BACKUP_DEST}/ && rm `ls -t *.tar.gz | awk "NR>$NUM_TO_KEEP"`) 2>/dev/null
# Alert succes
echo "Critical Backup Archiving Completed"
exit 0
CS01-HS's post in Auto .tar a directory was marked as the answer