Nationalism, politics can unite people just as well, and unite them long-term.
No it can't not on non trivial time horizons. Nationalism and politics require a common cooperative foundation to sustain them. Inertia can only holds things together for a limited time.
Similarly your stated purpose of competitive reproductive and genetic supremacy is barren ground to organise a society around. We saw this tried in the 1940s and the horrors it leads to.
You are correct that every human theocracy is and will continue to be terrible but that is a separate topic related to human ignorance, and lust for power.
We are not going to agree so I am going to give you the last word in this conversation.
I will leave you with a different book recommendation since you were not interested in the religious one. Here is a science fiction series related to the topics discussed above. It is totally free of any talk of God.
For those that like science fiction I recently read the Doom Star Series by Vaughn Hepner. It is interesting social commentary underneath a good story.
https://www.amazon.com/Star-Soldier-Doom-Book-ebook/dp/B003SNJVH4It envisions a dystopian future where humanity has terraformed and spread throughout the solar system and traditional religion appears to have has died out or been suppressed.
Humanity has splintered into various ideological factions. Earth is under the control of a stifling planet wide socialism. Mars and Venus are under the control of genetically engineered super humans who believe their superiority gives them the right to rule. Jupiter is controlled by philosopher kings who value only logic. Total laissez-faire capitalism dominates the outer planets and on the edges of the solar system a group seeks to create the ultimate controllable soldier by mixing man and machine.
As the story progress the various groups compete for dominance committing ever more horrific acts of evil that are completely justified by their various philosophies. It is quite clear that in this future humanity is in danger of extinction as the self-inflected horrors worsen and billions start to die.
The series is subtle social commentary the reader slowly realizes that while some ideological groups are better then others they are all pretty bad.
It is a vision of a future without God where religion in the form of various ideological and political constructs is very much alive and well.