I read this maybe 20 years ago(and it may be much older than that)... I don't recall if it was a short story, or a novel.
Things I recall:
The galaxy is largely populated by aliens.
Humans are rare, and scattered in small numbers across many worlds. They may be considered an ancient race. Despite the small numbers, they may command some sort of authority. I do not believe they have a "current" homeworld, more just residing on alien worlds all over.
Humans and almost mystical or revered to the aliens.
The location of earth or terra (don't recall what it was called in the story) is unknown to everyone. Considered mythological, really.
The protagonist is human, the story starts with him going to work in a local noble/official's court or office in a city. I don't really recall what his actual job purpose was. I seem to recall humans tended to wear monk-like robes. I want to say(not sure) that the noble/official's name started with the letter "J". The protagonist may have quit, or been fired, or banished, or sent on a quest.
At some point, the protagonist sets off to find earth
I MAY be confusing this point with an unrelated story, but I seem to recall at some point there are armies of humans from long ago being kept in stasis, and it may be implied that the result of letting them out is basically going to be galactic conquest.
The protagonist does travel to multiple planets in his search.
The protagonist may at some point consult with a library or librarian. I believe the librarian is female, but am not sure if she is human or alien.
I don't recall if Earth/Terra/whatever was ever found in the story. I seem to recall there's a cave(I may be confusing this with that same unrelated story mentioned above) at some point.
The protagonist does not have any sort of special or unusual powers or abilities that I recall, beyond simply being human.
I don't believe the tone was that of an action-oriented adventure; it was slower paced and more thoughtful, I seem to recall there were few to no instances of combat or weapons at least in the beginning parts of the story.
The protagonist is an adult. I'm not sure how old, I think maybe middle aged.
Humans may have been bald(whether by depilation or some natural evolution thing, I don't know.). I don't recall for sure, and may be confusing this point.
From digging around, I'm mostly sure it's not:
The Dumarest series, as aliens were definitely a thing.
The Roads of Heaven series, as the protagonist was definitely male.
Earthblood by Keith Laumer and Rosel George Brown, as besides the tone issue, the protagonist is an adult.
Asimov's Foundation series. (It's possible it might be set in the same universe, in the distant future? It did have a similar tone, though there was zero mention of psychohistory. And, again, aliens were in this story.)