I also think being a Starfleet Officer gives a person Federation citizenship and protection even if their homeplanet is not in the Federation
Simply migrating to a Federation planet (or space station, what have you), and setting up residence there might be all that is require to be considered a "Federation citizen." All the rights, restrictions, and responsibilities are yours.
In the same way, when Worf marries Jadzia, she joins Martok's house as well and becomes a "citizen' of the Empire, with all the rights and responsibilities therein.
I wonder if Jadzia would have been considered a second class citizen? Because, while she was married to a Klingon, she wasn't a biological Klingon herself.
There are Klingons with family houses, who are on the council.
There are Klingons with family houses, who are not on the council.
There are Klingons without membership in a house (commoners).
Non-Klingons living within the Empire. When Ancient Rome would conqueror a new land, it's residents became Roman citizens (like it or not).
The occasional rumored slaves.