I think this is an example of how some postmillennials (namely Reconstructionists) deviate from a biblical conception of the doctrine.
The “silver age” of a “Christianized” Earth before the consummation will be brought about by God’s grace manifested through the church’s faithful obedience to the Great Commission, not by the implementation of a given political system or strategy. Reconstructionists tend to focus excessively on the latter and seem blind to the fact that they (often unwittingly) communicate an ideal that is nominal. I see the kings’ repentance and acknowledgment of their ultimate Ruler laid out in the Psalms as teaching how even the mighty will embrace and be subdued under Christ’s dominion, not as an exhortation to focus on a “top-down” strategy (“if you get the state, you’ll get the people”).
That isn’t to say I think debating the “most biblical” form of government is futile. I am Establishmentarian, so I do believe magistrates are obligated to uphold both tables of the Law and that we should call them to do so even now, but my faith isn’t in the system - it’s in the Holy Spirit and in God’s promises. Apart from these two, even the “correct system” is doomed to fail.
I also believe we’re still in the early church, so most of us in this world are living in Babylons rather than Genevas.