God as I understand Him to be self-sustained, self-sufficient without need of anything. So what is God's purpose outside of recruiting His kingdom that in time will battle the dark forces that oppose the light of truth and good?
However God as I understand Him to be is capable of defeating sin and darkness on His own merit because He is God.
So wouldn't this indicate that "recruiting His kingdom that in time will battle the dark forces that oppose the light of truth and good" is not Gods purpose nor is it the intent behind our existence?
Why involve humans on the battle field of good and evil when God can restore the world to its former state at anytime?
Because God wants us to be good. To be good you have to choose good, to choose good evil must be an option. I like the example of AI, let's say I build a robot that help people who need it cross the road, would you say this robot is good? I wouldn't, because it is simply doing what it is programmed and designed for, it is incapable of doing anything else whether it wants to or not so at most you could say is whoever designed/built/programmed it is good. Now if I built a robot and this robot had a true AI that could make decisions for itself and it decided to help people cross the road, then you could say it is a good robot.
It is the same with Gods creation, if we were created only with the ability to do good, into an existence where there was only good it would say nothing about us, only something about our creator. I believe the purpose of this existence is to reveal information about ourselves.
Perhaps God wants reassurance that sin does not enter into the world by choosing the inhabitants of the new world from those who are forgiven and redeemed from sin?
As you said, God is self sufficient, He would have no need for reassurance and especially not an external one.
Can you imagine life without pain and discourse? What will we do with ourselves without challenges and tension, how will we grow and learn? Will any of these things be important in another lifetime?
Yeah I can imagine it a bit, I will probably jump off a cliff or two for fun.
The question of boredom in the afterlife is one I have considered but I don't think it is one we can really answer.. however if you have faith in God for so many things believing He has that sorted isn't a stretch.
Will overachievers live in harmony with slackers? Whatever, it.s not supposed matter anymore.
I think any two good people can get along
I think you are looking at this from the angle of "what's in this for God", in that sense I think the answer is that God is aiming to establish His right to rule. He is showing what happens when we try our own systems of govt and law. Even that however isn't really for God, He could make everything run like clockwork, this is all for our benefit.