You can use Proselyte + Prayer potions. Way more afkable and you will save lots of potions if you have an ancient mace. Special attack increases prayer points even above normal level, so as to save potions. And as for someone saying that Dharoks is afk, it's not. Some people have been using Mouse recorders to prayer flick, or auto clickers. It's not a guarantee you will not be banned, especially with the bot nuke at NMZ and other places lately. Hope my advice helped.
If you don't want to use prayer pots, then stick with Guthans. Although it will be afk for 6 hours, the xp will be way lower since it's Shared xp.