I agree that there is not much to do in the Sandbox Mode so far. Adding more quests and locations will be the main priority now that most of the basic systems are up and running. There should be more content in a couple of months.
Weapon durability is always a controversial topic. With a higher skill and better weapons it becomes less of a problem, but I see why it can become frustrating at the start.
I'll have to rebalance the arena equipment at some point, as there have been changes in the damage and health calculations, recently. But the balancing might stay a bit off for a while, as it is difficult to scale damage and character progression for an open-world game when there is just the starting region yet.
Thank you for the feedback and good luck with your own project.