For Benny Russell and millions of others, which would include my grandparents and parents, among other family members, roughly at the time "Far Beyond the Stars" took place, it wasn't just a struggling 'phase', that was a struggling life, a complete existence living under a system which told you were inferior and it was codified in law and custom and ruthlessly enforced if need be.
For Russell, IMO, it wasn't just an issue of writing a book, but also claiming a humanity that had often been denied him every day of his life, and also denied that to the people he knew and loved, for no reason except they weren't born the 'right' color. With science fiction, perhaps to him that was the one area where there could be an escape, that if one could read stories and imagine worlds filled with aliens, then the idea of a black captain wouldn't be so astounding. That readers might be inspired to think outside what society told them was possible for black people at that time (I would also argue today as well, but that's another conversation). This was more than just a job or an assignment. And if you compromised once, I can imagine they would expect you to keep compromising until by the end you had nothing left, your opportunity to create something memorable and special was gone, and things had not really changed. Just recently I saw an article where George R.R. Martin regretted allowing the publisher to not place his black main character on the cover of his novella Nightflyers, which has been corrected in some new editions. Black sci-fi author Steven Barnes also had some issues with a whitewashed character on the cover of his novel, Streethlethal. Both of these issues happened in the '80s, I believe, so that's not even as 'far' back as in Russell's era. (Barnes wrote the novelization for Far Beyond the Stars).
https://bookstr.com/george-rr-martin-speaks-out-against-white-washing-nightflyers
The wikipedia link to EC Comics goes over the controversy of their story "Judgment Day" which is similar to the struggles Russell had to face, and likely served as an inspiration for the DS9 episode.
https://en.wikipedia.org/wiki/EC_Comics