The Millennium is not symbolic of the afterlife as the after life will be in the New Jerusalem after this present heaven and earth pass away and the New Jerusalem ushered down from heaven where we will be with the Lord forever after. That does not happen until after God's great White throne judgement and Satan being cast into the lake of fire, plus all who have followed after the beast and its false prophet Rev Chapter 20-21.I tried to interpret the 1000 years as symbolic of Heaven, but Revelation 20:7 pretty specifically says, "After the thousand years are ended." You can't have something happen after Heaven. So while the number 1000 may be figurative--and frankly I think it is--the Millennium cannot be symbolic of the afterlife.
A key may be in a fad going on in the political climate John was living in when he wrote it. Even though John himself was imprisoned by Rome, there was this concept of the Pax Romana, or Pax Millennium. Octavius and Mark Antony had gone to war over the Roman throne not long before Christ. Pax Romana was the rallying cry espoused by Octavius' forces: or "Roman peace". They wanted to spread the "Good News" to all of Rome, that there will be 1000 years of peace from here on out, with Octavius as our emperor. Hence the term "Good News" as well (which we borrowed from to mean something else today). That Pax Romana ended up lasting more like 200 years, and John probably didn't even perceive it as started yet, but that's the idea. John was borrowing from a Roman fad of his time, and adapted it to apply to Christianity--the same way we did "the Good News". John, of course, never intended us to be the reader of Revelation--he wrote it to the seven churches of his day, who would have known what he meant. We just take what he wrote to them and apply it to our own lives.
You can't just read Rev 20:7 without reading the full context of verses 7-10 as this is speaking about the defeat of Satan. This happens before the New Jerusalem is ushered down.
Between the binding of Satan and being loosed scripture says 1000 years, but many parts of Revelation are symbolic in what John was shown in His visions. I see 1000 years only being a figurative numbering as I can't see God waiting a literal 1000 years after we are caught up to Christ in the air before casting Satan into the lake of fire and then ushering down the New Jerusalem after His final judgement of all that have died or still alive at this time.
Revelation 20:1-8 is the only passage in the entire Bible that the premillennialists have as the basis for the 1000 year reign. Rev 20:4 doesn't speak about a 1000 year reign of Christ it says by the vision John received "and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God and they lived and reigned with Christ a thousand years." It's not Christ that reigns 1000 years, but those who were killed for God's sake that reign with Christ 1000 years as 1000 being a figurative number, not a literal number as in Deuteronomy 7:9; 1Chronicles 16:15; Psalms 50:10; 90:4; 105:8 Ecc 6:6; 7:28; Daniel 5:1; 2 Peter 3:8.
I never heard of Pax Romana, or Pax Millennium so I researched it. We will see this history repeat itself when the son of perdition takes his literal seat in Jerusalem as we read this in 2Thessalonians 2:1-12 and Rev 13.