I was sure a fan of Pat Buchanan....Great analysis. I agree with all three of your points regarding ecumenism, Enlightenment values, and chiliasm.
Another problem with Christian nationalism is that it might promote the heresy of phyletism. The only historical precedent we have of a truly Christian nation are the Orthodox kingdoms (Constantinople, Orthodox Russia, Serbia, etc.).
In the Western world, I think the best we can do is to vote for libertarian candidates who would leave us alone to practice our faith, instead of imposing Covidism or LGBT nonsense on Orthodox Christians.
Ideally, paleo Mises-style libertarians are best, since they tend to dislike abortion, mass migration, and globalism.