I believe the entire book concerns the future.
The first three chapters pertain to what happened right then.
I believe the seven churches are churches that will exist during the trib. The "flavor" of the letters is Jewish, not Christian. In a couple of them, Jesus warns that they must "endure to the end" to be saved". Christians have the guarantee of salvation. The letters are written to the "angel of the church at xxxx". The epistles to the Christian churches are written to the churches, not to the "angel of the church". The use of other terms, like candlesticks, lampstands, synogogue, are decidedly Jewish, and not Christian.
And even with the seals, if you compare them with the Olivet Discourse, the first five seals were operational from the first century and are progressively getting worse. The 6th and 7th seals are reserved for the future (which may not be too far away).
"The things which have been" = the development of apostasy in the Apostolic churches
"The things which are" = the current status of the seven churches in Asia Minor (chapters 1-3)
The things which shall be hereafter = prediction of future events (chapters 4-22).
Rev 6:
1) And I saw when the Lamb opened one of the seals, and I heard, as it were the noise of thunder, one of the four beasts saying, Come and see.
2) And I saw, and behold a white horse: and he that sat on him had a bow; and a crown was given unto him: and he went forth conquering, and to conquer.
^^ who was on the first horse, and when did it happen?
3) And when he had opened the second seal, I heard the second beast say, Come and see.
4) And there went out another horse that was red: and power was given to him that sat thereon to take peace from the earth, and that they should kill one another: and there was given unto him a great sword.
^^ While you could argue that the world has been at war in some form since the first century, I believe it's speaking about the future, when wars will be much more apparent and encompass the world.
5) And when he had opened the third seal, I heard the third beast say, Come and see. And I beheld, and lo a black horse; and he that sat on him had a pair of balances in his hand.
6) And I heard a voice in the midst of the four beasts say, A measure of wheat for a penny, and three measures of barley for a penny; and see thou hurt not the oil and the wine.
^^ When has there been a world-wide famine?
7) And when he had opened the fourth seal, I heard the voice of the fourth beast say, Come and see.
8) And I looked, and behold a pale horse: and his name that sat on him was Death, and Hell followed with him. And power was given unto them over the fourth part of the earth, to kill with sword, and with hunger, and with death, and with the beasts of the earth.
^^ When has 1/4 of the world's population been killed?
I believe the book of Revelation provides closure to all the things prophesied in the OT about "Daniel's 70th week", the "day of the Lord", the "time of Jacob's trouble". AKA the great tribulation. While we can certainly learn a lot from it, it is not addressed to Christians. We will have been raptured to Christ.
But I do agree with you that it might not be too far away. I believe time is short. We'll see.