I do mine through:
https://eservices.capetown.gov.za/irj/portal
you have to Apply for the City to enable the ability for you to renew license online.
but once thats done, you enter in the make of your vehicle, Your License plates and same day, an agent will email you the amount due.
Once paid, takes about 30days for your license to be delivered via post.
So i do it the month before the expiry date then i get the discs 1 or 2 days after the expiry date.
Long wait, but no queues is always a plus.