Yep, send them a payment for your monthly amount x12 and add a bit for the “credit card” surcharge.
Optus will automatically deduct any credit amounts first from your account. CHeapest option is to set up a direct debit on your account (This avoids about $25 in payment fees). Then as suggested just pay your annual bill in one transaction. Each month Optus will deduct from this amount before activating the direct debit. PLace a reminder in your calander for when the term is nearing renewal.
With postpaid mobile accounts going overseas for 4-6 months and using overseas sims mostly, but not wanting to change the OZ accounts , we put 6 months plus a bit onto the accounts at an optus shop and it deducts the monthy payment from the credit each month as soon as it is due. No message sent or action required.
It should work for you, but Optus accounts is a dice roll.