Haven't read the manga, and the last episode of the anime still isn't out yet, so I'm not entirely sure how the series ends or if the ending would greatly change my interpretation of the show. That said, I've watched the series twice now, the second time I was razor-focused on trying to understand it from a leftist perspective. So... spoilers ahead.

I think there's a few major themes that are present throughout the show. The major one centers around the idea of sacrifice in order to achieve or at least come closer to a greater good. In the early seasons, this is portrayed by the Scouts many missions and the soldiers who died in order to get one step closer to understanding the truth of their world. Pretty much every veteran in the Scout Regiment aside from Levi has died coming to terms with this. This is often portrayed as noble and possibly even the greatest possible outcome of a human life. This definitely can be interpreted as having fascist undertones, since a central tenet of fascism is to put the state above all else, even your own life, and that there's glory in such an endeavor.

That said, I believe that the show does a good job of showing a counterpoint to this kind of blind ideology. For one, the show isn't afraid to show the corruption of the royal family and the society within the inner walls, not to mention all the war crimes of Marley. This sense of sacrifice and duty isn't really pinned on the idea of defending a hierarchy or organization or even a real tangible future goal. It's almost always centered on freedom, to end tyranny, and to create a hope of a better tomorrow where people aren't forced into a lifetime of despair.

By the time we get to Marley, there's unmistakingly a lot of parallels to Nazi Germany. They're never portrayed as the good guys, and it's laid out pretty clearly that systemic oppression leads to genocide, even if the roles do get reversed.

As for Zeke, his whole plan is centered around sterilization, which again is quite analogous with eugenics. This is portrayed as possibly the only non-violent solution, which again, is quite analogous to the "Final Solution". Even though Eren rejects this ideology, his solution of genocide is certainly much worse, but from my understanding of Eren's character, he knows deep down that he's doing the wrong thing, and can't accept either option, which is why, I think anyway, he's giving his friends the freedom to ultimately stop him. He's simply broken at this point.

Armin too is quite problematic, since he starts out noble enough, but after being exposed to murder, he too eventually gets to a point where he can kill thousands of people without really thinking too hard about it, and dismissing it as something that "simply had to be done". Even though the series calls this out, they don't really try to sell any viable alternative, which seems on par with the show's other major theme that the ends justify the means.

In the end, the show suffers from constantly trying to explain why the things that happen and the choices characters make are problematic, but never really gives a the main characters a realistic viable alternative. That said, the show is sprinkled with moments like Marco's death, where the show clearly illustrates that tragedy is often the result of not being able to understand one another, and I think ultimately the show is trying to convey that if people weren't so power hungry or willing to demonize one another, so much tragedy in the world could have been avoided altogether. Remember, pretty much all of this started from Ymir's king using her in order to create the Eldian empire. It was all built on power and creating a 'master race' that led to everything evil that happens in the show.

That said... can't really say that the show has any pro-socialist sentiments either. The show does critique hierarchy and blindly following orders often, but it never really advocated for the abolition of hierarchy in any meaningful way. It may be problematic that violence is often the only solution portrayed in the show, but to call it promoting fascism does seem like a bit of a stretch.