This is a Monty Python level parody, played deadly serious.

"Read my points and try to shoot them down."
***
OK Sir, I'll try.

"Even the Cimmerian culture is presented as a flop. They get wiped out."
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Indeed, in Milius' film, Conan is represented as 'The Last of the Mohicans... er, Cimmerians'.

"Arnold's Conan is an utter moron."
"Conan's father was dead wrong about the Riddle of Steel, and the Father's Sword snapped in half like a toothpick when it clashed against the Atlantean".
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I think any perceived "moronicness" is less due to the writing than to Arnold's limited acting capabilities at the time (although he was, of course, physically perfect for the part).
Conan himself is not just all brawn, but also something of a philosopher -both in the film and books-, who reflects on the world and his own condition.
Milius's film shows very well how Conan's philosophy, understanding of his own way of being-into-the-world, ideas about religion, women, etc. all evolve throughout his adventures:

How he gradually learns that neither Steel (his Father's philosophy -wrong indeed, as you pointed out-), nor Flesh (Tulsa Doom's philosophy) move the world, but rather the Will that wields both (he's seen holding both his father's broken sword and Doom's severed head in the finale, and tosses both; he's able to subdue the entire cult and its remaining followers through his sheer presence, fearlessness and willpower, without weapons or political power).

How, as a child, he blindly accepts his father's tenets about religion (the opening monologue about Crom), then comes to thinks of one's faith as something to oppose and defend against other faiths (“Crom laughs from his mountain at your Four Winds!”), but finally understands that the purpose of God and religion is to serve men, not pit them against each other (Conan's prayer to Crom before the Battle of the Mound: "Grant me one request... and if you don't listen, then to hell with you!").

(continued...)

(...continued)

How he goes from being a lustful beast used as breeding stock, to understanding the potential dangers of casual sex (the encounter with the witch), to understanding the profound attachment and meaning of commitment to another human being (Valeria, whom he treats as his equal).
At the end of the film, Conan doesn't try to take advantage of the rescued princess who worships him and kneels before him: he instead gives her back her dignity and takes her to her father.

How he goes from the simple, belligerent worldview and politics of a thug (“Crush the enemy, see them driven before you, and hear the lamentation of their women...”), to a materialistic world view (his “Tony Montana phase”, so to speak, where he literally drowns in his soup, covered in jewels and gold -and of course Oliver Stone co-wrote the screenplay), to a higher understanding of morality, honor and a more philosophical view of the transience of existence (“No one, not even you, will remember if we were good men or bad, why we fought, or why we died. All that matters is that few stood against many.”)

And when Conan is chased by feral dogs after having gained his freedom, he enters an abandoned tomb as a frightened brute still in chains, but emerges from it a free man, with purpose and a sense of History as well as of his own place in the world, after his encounter with the remains of the Old Warrior King.