I think this was the key piece I was missing, if I'm not mistaken:
"To pick only full backup jobs that occur at a particular frequency, click Select Full Backups at frequency, and then select the time period at which to pick jobs for backup copy:
- For Daily or Hourly full backups, select the time at which the day begins.
- For Weekly full backups, select the day on which the week begins."
If I understand correctly, the Automatic Aux copy checks every 30mins and ONLY when it sees this Weekly Full Backup (Tape) job / setting it kicks off the backup to Tape - Have I got that right?