Series of novels involving a galactic empire in slow decline
This is a long-shot; I only remember a few details from two novels I read as a child, by the same author. This means they were originally published in the late 1970s at the latest, but I would guess they were older than that.
Both take place in a galaxy-spanning empire that is in decline; this decline is apparently irreversible, and has been going on for a very long time, on the order of centuries or millenia, as the central government's resources diminish and as there are rebellions on the peripheries. Technology is advanced, but stagnant. The central government is referred to as "Central Command", often abbreviated to CC. Mutants, and "greenies", are discriminated against, or worse. People from many cultures are considered "barbarians" and are second-class citizens, with restrictions on their occupations; one of the few that is open to "barbarians" is that of mercenary.
In one book, the story centers on the crew of a military vessel, which crash-lands on an uninhabited but pleasant world, which turns out to be Earth. In the opening passages of the book, members of the crew reflect on their losses to attrition in a long series of conflicts. 1012
One talks about an android, who'd helped them in one crisis, and another says, "That android was great! When did we lose him?", to be reminded of another battle. At the end of the novel, the protagonists meet the crew of another ship that had crash-landed on Earth; they decide together to turn their back on Central Command and settle on Earth. It's left ambiguous whether humans had originally come from Earth and abandoned it, or whether these were the first humans to settle there.
The other book is also set on Earth; it seems to take place sometime in the near future. I don't know how it relates in a timeline to the other novel I described, whether long before or long after. At some point, there was a reference to the city of Sacramento, as an agricultural center. Earth is considered to be under the oppressive authority of Central Command, but is a "barbarian" world. The protagonist, early in the novel, sees a sunlit wall, and thinks he'd like to stretch out on it, but does not, because that would identify him as a "greenie". It isn't really explained what a "greenie" is. 1012