Your take on the New Jerusalem being the the state church establishment that arose out of the end of the pagan Roman empire (at least, that’s what I assume you are saying), is a perspective, but it runs up against some major difficulties, apart from whether it is historically credible.
I think the references to those outside the New Jerusalem continuing to have a separate existence from those within the New Jerusalem as a fulfilment of Zechariah 14 is overdone, the resonances of Revelation with Zechariah notwithstanding. For instance, “the kings of the earth” (21:24) come into the city, but there is no mention of an annual visit of them or of “the nations”, as in Zechariah. They might equally have gone in and stayed there! There is anyway no longer a temple for them to go up to annually with its old covenant feasts, to celebrate the Feast of Tabernacles. There is no longer a city that can be visited annually.
It’s the absence of these elements of Zechariah which make the finality of the vision of 21-22 compelling. Likewise the exclusion from the city of sinners (21:8, 22:15) does not suggest their continuing separate existence. Rather, what has happened to sinners in 21:8 (the lake of fire/second death) is logically the fate of sinners in 22:15 also, not a continuing existence. This is a future reality, not a first century phenomenon.
This raises another question about your interpretation. There is no mention of the lake of fire (19:20, 20:14-15, 21:8) in your opening piece, which presents the New Jerusalem of 21-22 as the continuation of the church in history. In your exchanges with Davo, the lake of fire has become the end of history: at least, I think that’s what you are saying about its appearance in 20:11-15. If the New Jerusalem is the church in history, then the realities of 20:11-15, which echo the fate of the beast and the false prophet in 19:20 and the fate of sinners in 21:8, as well as the end of death in 21:4, have already taken place. That seems unlikely, even allowing for apocalyptic hyperbole.
These are strong arguments for placing 21 and 22 in the future. However, Revelation being what it is, there is a case to be made for the New Jerusalem and the ultimate future realities of 21 and 22 already having arrived in part. This would make sense of the power of the ministry of Jesus in the gospels, and account for the effectiveness of the early church in Acts, and the meaning of the letters. There is concrete evidence in history of what is to be completed in the future.