You’re just dickering on the cutoff date being 1776 or 1787.
Since there was an Articles of Confederation prior in between, I think most would argue that the United States came into existence when the Constitution was agreed upon.
The men who signed the unanimous Declaration declared within the document that they were the representatives of the united States of America. Were they dreaming? Are you claiming to have more knowledge and understanding than they had?
Also, the Articles of Confederation claimed for the inhabitants of each State the rights and privileges of citizens of all the States in the confederation. So, all were citizens of the States, and not subjects. So, any which way you look at it, people eligible for the office of President during the first presidential elections had to be citizens. And the same for subsequent elections as long as men wanting to be president were citizens at the adoption of the Constitution. If a person who wanted to be President was not alive at the adoption of the Constitution, he needed to be a Natural Born Citizen, a more restrictive class of citizenship.
From this point in the debate, you are, in effect, saying the NBC definition established by the “Law of Nations” was not recognized by the founders as they debated the NBC clause in the Constitution. That is false, as you can discover by reading notes of those debates. If you haven’t studied those contemporaneous notes, that should be your next course of study, I would suggest.