Hmm.
Is there any reason to think that God, if God exists, would want 100% of people in the world to believe in Him?
Yes. There are many reasons. Each person is different in their need for god. Mainly for reverence, understand life, ones place in the world, ones relationship with it, and/or et cetera.
If God wanted everyone to believe in Him, what do you think God would do in order to accomplish that?
It honestly depends. No believer in an abrahamic god has explained the nature of god to where I would know what/who he is in order to believe in him. But I dont know how god wants everyone to believe in him.
Maybe first defining himself so that we know he is the one and only god humans are "asked" to believe in. Anyone can name characteristics to a mystery.
God doesnt need to be objective or scientific. I do find it useful that humans have, at the core, the same type of experiences. For example, when neurons go crazy, we have seizures. While each person has different types of seizures and illnesses that mirror seizures, we all can pin point the cause which are the nerves and neurons.
If you cant explain god, we see the "symptoms" and interpretations of those symptomns but we dont see the cause or nature of origin. That means the symptoms can be from anything.
God is to vague.
In order for us to believe, he has to be at least understood to have some sense of belief. It has to go beyond feeling. It needs to be distinct if you want humanity to believe. Fortuntately, life doesnt work that way. Each person has their own definition of god.
So it makes it hard to understand who we (everyone) are to believe if we dont know anything about it.