Interesting question. At least the term is not "Comrade." Yet.
In your example, Thomas Jefferson probably by habit initially used the term "subject" in his draft of the Declaration of Independence since the colonists had indeed been subjects of the king. Jefferson also used the term "natural subject" in his 1783 words describing English law that I posted to start this thread. But now the colonists were going to be free of owing allegiance to a king, and they would possess rights and privileges and the ability to vote for the people in charge of their government(s). So, a term other than "subject" was appropriate.
You can find similar descriptions of "subject" and "citizen" in the 1828 Webster's Dictionary [Link].
I imagine "citizen" originally meant inhabitant of a city. From the internet [Link]:
Origin of citizen
12751325; Middle English citisein < Anglo-French citesein, Old French citeain, equivalent to cite city + -ain -an; Anglo-French s perhaps by association with deinzain denizen
The French also switched to the term "citizen" for describing themselves during the French Revolution when they overthrew their king.
Citizen appears to be derived from "City-Denizen." Dweller in a city. In fact, I have searched Blackstone and Shakespeare, and in every case that I found, every usage of the word "Citizen" refers to city dweller, as in "Citizens of London." In fact, if you look at old English dictionary's of the time period, they say that "Citizen" means city dweller.
A dictionary of the English language. by Samuel Johnson, 1768.
Cit. [contracted from Citizen] 1. An inhabitant of a city. 2. A pert low towniman.
Citizen. f.[citoyen Fr.] A Freeman of a City. Raleigh 2. A townman; not a gentleman. Shakefp3. an Inhabitant. Dryden
"The Royal Dictionary, French and English, and English and French" 1764
Citadin, S. M. Citadine, S.F. (habitant d'ene cite) a Citizen
Citoyen, enne, S.(bourgrais, habitant d'une cite) an Inhabitant, or Freeman of a City.
I've read other dictionaries of the period, and they say the same thing. "Citizen" is originally a French word, and in the English usage of the time period meant "City Dweller." It also meant "City Dweller" in France during this time as well.
So how did this word that means "City Dweller" come to mean member of a nation state?
Here is a clue as to how this happened.
"Sense of "freeman or inhabitant of a country, member of the state or nation, not an alien" is late 14c."
Hmm. What happened in the later part of the 14th century? What caused this word that meant "City Dweller" to change so that it meant member of a nation state.?
I was hoping for some commentary regarding my last message to you in this thread. Believe it or not, i’m going someplace with this, and you might find it quite interesting when you learn where this is going.