Fascist nerds are kind of unique in that they're already incorporated the intensely negative self image into their ideology. They don't necessarily have the belief that fascism is helping anything in society or that they are a positive force in the world. A lot of them want to die.

So an effective satirical 40k novel would have to do something like getting the reader invested in a story that isn't just endless grimdark space warfare to the point the reader actually wants to escape from the baseline fascist ideation in the franchise. Reinforcing that there are no "good guys", there is nothing but warfare, the galaxy is just endless suffering, etc. All of that is perfectly in line with their world view. Like it is extremely telling that the chud nerd self mascot was never Chad. It's Pepe, the doomer, or wojack.

Edit: Now that I think about it, Moderan is basically what 40k would be as more effective satire. Moderan also stars a bunch of super soldiers in a far future that consist of perpetual warfare, it also has a lot of similarities with the typical space marine story where you start off with a human and see the transition into a cyborg killing machine. The difference with Moderan is that it feels genuinely painful to be trapped in that world with those people.

It feels awful to be trapped within the narrations of a mind that can't conceive anything outside of violence and hate. It feels awful to be trapped in the book's world because fascism has destroyed human culture to the point where there isn't even an aesthetic appeal to the omnipresent violence. So when the book hints at any kind of escape back into humanity, you lurch for it.