The real reason FSM upsets the fash is that it increases the depth of 40k
That's not what I want. Refer to my previous example - it's alright not to have FSM because the Imperium is a sexist hellhole, but only if you're honest in that portrayal instead of making a neat little Lore Knot to dodge the question. Because it's the questions that create depth and it's the questions that fash fans don't want portrayed, because what they want is an uncritical transplant of Nazi propaganda since it's harder to get away with enjoying something that's got a swastika on it than an Aquila.
As an evil upset frothing-at-the-mouth fash I would like to say I think FSM actually lower the depth of 40k, what with making actually unique organization of Adepta Sororitas way less, well, unique, and also would make Marines less unique, I guess. Unless one is a person who violently screams about females being 1:1 the same as males, I don't see how that would be a good thing. Heck, the push for having FSM would be sexist because by forcing females into an exclusively male organization which has very similar, if not identical in many respects, goals that a exclusively female organization(Adepta Sororitas in this case) also has, you are essentially saying that the female organization is not good enough. In other words, that'd be like saying women should be like men because men are better.
Similar themes would be the Imperium's role in and desire for permanent genocidal hostility between aliens and humanity even though it is demonstrably unnecessary,
Imperium often trades with and somewhat supports alien races that pose no immediate threat because they simply do not have resources to go and kill everyone on their borders. Xenos that aren't an organization separate from Imperium are usually not kill-on-sight in Imperium proper even, though there is very much deeply ingrained speciesism, which mainly comes from the fact for longest time humanity was on the receiving end of the "die Alien scum" stick, with communities that actually worked together being rare and in-between. The desire to genocide otherwise peaceful races comes from the Imperial "Manifest Destiny" feeling of ownership and responsibility for entire Galaxy, and that desire is demonstrably necessary, since to survive and emerge victorious against not only Chaos, but also Tyranids and other races that are very much more hostile and genocidal, Imperium would need all the resources it can get.
or the fact that daemons only even exist as a reflection of mortal cruelty.
Daemons and other apparitions of evil existed way before humanity ever got off Earth, and we have to thank War in Heaven, Fall of Eldar and other similar fun times for that. Does humanity have a hand in Warp continuing to be shit? Yeah probably. Did humanity have a choice to make Warp not be shit? Not really, unless you count one of Horus Heresy alternative outcomes where Horus (somehow) kills absolutely every living being in Galaxy as a viable choice.
A good example is the Imperium's role in humanity's survival. The Imperium simultaneously has protected humanity from existential threats and also ensured humanity's extinction, and that right there is something that you need an A+ author to even begin portraying in a digestible form.
The Imperium has protected humanity from existential threats that would have made it extinct long ago, and the "ensured" part is arguable, both in-universe and outside of it. Having a very minimal chance is way better than no chance at all.
It's weird, I've always seen WH40K as a cartoon making fun of fascism. It's not really subtle? The eagles, the cult of personality/worship of a lie, the embrace of death/heroism... the freakin death korps? The intolerance of mutants, the special police hunting down anyone who goes against the imperial creed?
The problem here is that WH40k is way more complex than that. Eagles were a symbol of, well, hope, the worship might be the only thing stopping the whole thing from collapsing, hunting down of mutants is not because they're different, it's because they're in fact, Chaotically tainted and basically a given that the Chaotic part would cause severe problems - better example of racism would be abhumans like Ogryns (or Ratlings or so on), who are different due to non-Chaotic reasons, and thus they are accepted in Imperial society, THOUGH that's where the real racism kicks in, since people dislike them for being big and dumb. Death Korps didn't ask to be like this, and them being like this can be traced back to when actual fascists decided to show a middle finger to Imperium. Also, as a tangent, above examples aren't even really just "fascist", they're very much present in other forms of governments, though mostly autocratic, just like Death Korps aren't a play on Nazis, they're a play on how fucking shit WW1 was, on all sides.
And the above is portrayed 100% evil, even on the battlefields,
Heroism and embrace of death is not inherently evil?
unless you take extremely narrow snapshots of what's going on. I guess the Emperor is morally complicated, but the Ecclesiarchy has completely abandoned everything he stood for and is just evil.
Ecclesiarchy isn't "fascist" even, they're church with nearly absolute power, with all the bad and good portions of it, even if the bad ones outweight the good, and the good are about never shown in most WH40k media - Ecclesiarchy runs hospitals, schools (including Schola Progenium (which you can argue are evil, but people who leave them usually end up pretty well for themselves, which is way better than dying on the street as an infant)), heck, Adepta Sororitas Militant Orders are just a fraction of the whole organization, which includes medics, scribes, advisors and essentially fucking diplomats.
Anything that portrays life in the Imperium outside of directly smiting daemons shows the injustice, intolerance, and general *suckiness* of life under fascism. It's gone from amusing to strange to kinda worrisome that people don't immediately see that. But I think that's because most casual "fans" only really look at the glorious battles. Even fascists should look at the Imperium and think "It wouldn't be that bad!", not "It doesn't look so bad."
Most casual "fans" only really look at Imperium and see "Space Nazis". Setting aside the argument about if it is really "fascism" or not (it's not, but I presume the word is just used as a stand-in), the injustice, intolerance and *suckiness* is just the most commonly shown side (because, it's, well, not mundane), you'd have to look deeper to find positive parts, which is very rarely done. The biggest problem is that Imperium has long ago moved away from being purely grimderp where everything was shit so it could drive a point home. It's much more complex since people realized a society that's just evil, has some logistic problems when it comes to how it functions. This isn't to say that Imperium is somehow now excused of everything evil, and is actually a purely positive force (though still maintaining that Imperium is good, since it's the only meaningful faction that promises more than death/tentacle rape/slavery for humans), but I believe looking at it and seeing a purely negative force is pretty much as naive as looking at it and thinking it portrays and promotes "fascism" as a positive thing.
Remember, too, that codices kind of need to plug their armies in fluff, or at least make them cool. For the Imperial factions, that means making fascism cool.
Just making humanity look cool is apparently passe, what with everyone finding Blueberry flavored "fascism" somehow more positive than regular "fascism".