Would life be worth living if there were no challenges to overcome?
Why wouldn't it? I have no challenges now except the ones I create for myself to meet some need (volunteering, learning, stimulation). That's the life we aim for - the one where we feel safe, secure, loved, have leisure and freedom from want, anxiety, fear, loneliness, regret, shame and the like. We don't come to that knowledge except through trial-and-error, which means making mistakes and learning from them, but that's the human condition, since we don't have the powers of a god to give ourselves or our children that wisdom by infusion.
Would people appreciate freedom without a knowledge of what slavery can be like?
Do you think they need to have been a slave or seen a slave to understand that having one's freedom, labor, dignity, and children stolen from him and being beaten is undesirable, and doing it to others immoral? Here would have been a great opportunity for a tri-omni interventionist god to program people to feel instinctively about strangers the way most do about their children. But that didn't happen. This god didn't condemn the practice of owning people, but rather, allegedly gave advice on how to do it properly and to what degree one could beat a slave before it being condemned. And I'm discussing chattel slavery here, not indentured servitude, which is a time-limited, consensual business relationship rather than what I just described above.
there's a benefit to struggling to achieve a worthwhile goal.
That doesn't mean that we're better off making mistakes if it were possible to prevent them as a tri-omni deity ought to be able to help us do by creating us with a mature moral sense where we had no desire to steal, betray, demean, etc.. I was not born with any of those. I have them now. I acquired them by making mistakes, my conscience punishing me, and learning to do better. As I indicated, that's the only way to get to that place given the human condition and the apparent lack of divine intervention. But if this tri-omni deity were to exist, one could be born with that knowledge.
It's pretty hard to make a case for a tri-omni deity ruling our world. One needs to make things that can better explained by there being no such deity seem to be the choices of a benevolent god who could have made them otherwise. That's divine command moral theory. Good becomes defined by what it is believed that the deity says, does, and wants, not his own (endogenous) moral intuitions, and that leads to the kind of comments you made here. This is the inherent risk in substituting somebody else's moral judgments for one's own - received morals. Don't you feel a twinge of cognitive dissonance trying to argue that we're better off having to make mistakes to come by this knowledge? I say that there is no other way to learn these lessons in a godless, naturalistic world, and so yes, challenges and struggles are necessary to acquire this understanding. But the believer in the tri-omni deity has to defend that being the design of a good god who could have made things otherwise.