That's actually a very important observation, and I think it's one of the places where many prophetic systems end up wrestling with the text.
If you read Revelation in its straightforward sequence:
Revelation 19: Christ returns.
Revelation 20: Satan is bound, the thousand years occur, then Satan is released, there is a final rebellion, and finally death and Hades are cast into the lake of fire.
Revelation 21: John sees a new heaven and a new earth.
That sequence alone is why many interpreters, including most premillennialists, place the Millennium before the new heaven and new earth.
Revelation 21 comes after death has already been destroyed (Rev. 20:14).
The difficulty you're noticing is that Revelation 21 contains language such as: "He that overcometh shall inherit all things" (Rev. 21:7).
At first glance that sounds like an ongoing call to perseverance.
But notice that Revelation has used "overcome" language from the very beginning (Rev. 2-3).
It doesn't necessarily imply that people are still sinning and dying inside the new creation.
It may simply be identifying those who had overcome through faith and are now inheriting the final promises.
What makes the new creation difficult to place inside the Millennium is passages like: "And there shall be no more death" (Rev. 21:4).
And: "Neither sorrow, nor crying, neither shall there be any more pain" (Rev. 21:4).
And: "There shall in no wise enter into it any thing that defileth" (Rev. 21:27).
Those statements sound very different from the typical dispensational description of the Millennium, where:
mortal people still exist,
nations still exist,
people can still rebel,
Satan is later released,
and a final rebellion occurs.
It is difficult to fit all of that into Revelation 21 after John explicitly says there is no more death and no more curse-related corruption.
So if you're asking for my honest assessment, I think your earlier instinct was probably correct.
Revelation 21 reads much more naturally as the eternal state following the defeat of Satan, death, and Hades than as the Millennial Kingdom itself.
The interesting question then becomes: how do passages like Isaiah 65 fit?
Isaiah 65 appears to describe conditions where:
people still die,
nations still exist,
yet there is extraordinary blessing and peace.
Many premillennialists place that in the Millennium.
By contrast, Revelation 21 describes:
no death,
no mourning,
no pain,
no curse,
no enemies left.
That sounds like the consummation of 1 Corinthians 15: "The last enemy that shall be destroyed is death" (1 Cor. 15:26). "That God may be all in all" (1 Cor. 15:28).
In other words, the connection you've been making with "all in all" is actually significant.
Once death is gone, the kingdom has reached its final goal.
That's why many readers see Revelation 21-22 as describing the state after Christ has finished subduing every enemy.
So I think the tension you're noticing is real.
If Revelation 21 is the new creation after death is abolished, then it is difficult to see it as a place where people are still dying, marrying, repopulating, and rebelling.
Those features seem to fit the Millennium far better than the eternal state.
