Let's be clear here. I'm an Amillennialist, so I don't view the thousand years the way you do. I believe that the figurative thousand years began after Jesus was resurrected which is when scripture says He began to reign over all things in heaven and on earth (Matthew 28:18, Ephesians 1:19-23). So, in my view we are in the kingdom of God, which Jesus current reigns over at the right hand of the Father, right now in a spiritual sense.You are asserting that the 1000 year kingdom is what they call "the eternal state"?
Colossians 1:12 Giving thanks unto the Father, which hath made us meet to be partakers of the inheritance of the saints in light: 13 Who hath delivered us from the power of darkness, and hath translated us into the kingdom of his dear Son:
Romans 14:17 For the kingdom of God is not a matter of eating and drinking, but of righteousness, peace and joy in the Holy Spirit,
I'm saying that the kingdom of God and kingdom of his dear Son are the same kingdom and we're spiritually in that kingdom now with Jesus, who is God, as our King. It's not as if we are in two different kingdoms right now.
So, in my view, even though we are in the kingdom of God now with Jesus reigning over it at the right hand of the Father, we will inherit the kingdom of God in its fullness when Jesus returns in the sense of inheriting it with immortal bodies, which we obviously do not have yet. And the kingdom will no longer exist in a place where the kingdom of Satan also exists and it will be free of sin and death at that point.
Since Matthew 25:31-46 talks about believers inheriting "eternal life" in "the kingdom prepared since the foundation of the world", that is when I believe that believers will inherit the kingdom of God with immortal bodies because inheriting the kingdom of God with immortal bodies implies that we inherit eternal life (eternal bodily life with immortal bodies) in the kingdom of God at that time. So, I see a direct connection between 1 Corinthians 15:50-54 and Matthew 25:31-46 (as it relates to the sheep/righteous).