Games

The book makes it a lot less handwaveable by giving the clendathu a much more pronounced civilization ane culture

Meanwhile the movie literally has a scene with one of the bugs mutilating a cow, and it's made up as poking fun at fascist propaganda, but it's also designed, advertently or not, to propagandize to the audience outside the setting too.

I don’t think Starship Troopers really describes an utopia

I disagree. Rico's family was quite well off. They lived a very comfortable life, and his dad didn't see any reason to go to the effort of gaining citizenship, because it earned him nothing more than what he already had. They had basic rights, they just weren't citizens with the right to vote. Rico joined because he had a more civic-minded group of friends that was joining. TBH, Rico isn't shown as being particularly bright; when he did his aptitude tests, mobile infantry was the very best that he was qualified for, and... That wasn't really that great. (That would be like getting a 36 on the ASVABs right now; juuuuuuuuuust enough to get into the Marine Corp, and not high enough to do anything other than the most basic jobs.) As far as a human population goes that we see in the story, there isn't any obvious underclass, which is a requirement for fascism. Yes, there were non-citizens that had no vote, but there was always a path for them to get their franchise, if they were willing to put in the effort. (As far as I recall, there wasn't a specific requirement that you serve in military; given that Heinlein goes out of his way to say that a position would be created for someone that was profoundly disabled, I think it's pretty clear that it was never just military service. But Heinlein was a military vet, and he was writing science fiction for a YA audience at the time, and military service was more exciting than, say, civil engineering.)

Moreover, Heinlein goes out of his way to say that no one that is currently in their term of service has the franchise. By choosing to become career military, Rico is giving up the ability to exercise his franchise unless and until he retires. Fascism is often--if not always--characterized by military control over large areas of civilian life. But if the military has no direct political power, that's sharply undermined.

And the democracy-accustomed reader is expected to understand the horror of these practices,

...But it was a democracy. It just wasn't a birthright democracy. Every person had to affirmatively choose to work for citizenship, rather than citizenship being granted based on where you were born, or who your parents were, but the right to work towards citizenship was afforded to everyone regardless of parentage, etc. I think that's a pretty positive thing, TBH. I think that the US would likely be a lot better place politically if everyone had to be personally invested in the system, and if everyone had to actually pass civics and history tests before they were allowed to vote. I know, I know, it's crazy, but maaaaaaaaybe a basic understanding of civics would mean that someone like Trump never would have gotten elected in the first place, since he's so bent on undermining and destroying the entire system.

I think that Heinlein gets some things very right; people value a thing much more if they have to put in some kind of personal effort to get the thing than people who simply have it handed to them. I also think that he gets some things very, very wrong, like his ideas on corporal punishment. (Which, TBF, were not that out of line for 1959.)

And this, in my opinion, is Starship Troopers’ greatest and most unique trait.

I think that you haven't read very much Heinlein if you think that he was writing satire at this point in his career. Stranger In a Strange Land had elements of satire, and Job was absolutely satire, but things like The Moon Is a Harsh Mistress is very straight forward. Heinlein is remarkably consistent in a lot of his writing; he's largely libertarian and believes people should be able to be self-reliant, but he still mostly believes in the gov't as a positive force. He's deeply distrustful of religion though; any time religion shows up in his books, it's a net negative.

We're also not given any circumstances behind this war. We can't draw any definitive conclusions about it.

Heinlein was a WWII veteran, and WWII was where the concept of 'total war' was pioneered. (And later demonstrated to be a pretty bad idea, but that's still not accepted by everyone.) We firebombed Tokyo and Dresden, knowing that it would cause massive civilian casualties, with only the barest military excuses; civilian casualties were the point, because we believed that it would break the will of the people of Germany and Japan to keep fighting. We can look back now and see that this was a dumb idea, esp. since Hitler was doing the same thing to England, and it was stiffening the resolve of the English. But unless you argue that the Allies were fundamentally fascist during WWII--which seems plainly false--then it's not reasonable to argue that the idea of total war was a fascist idea.

At a minimum, we know that it's a failed idea now.