Something interesting about the movie, and maybe there is a translation screw up but the official english dialogue claims Sailor Chaos was defeated and then Sailor Cosmos was so blackpilled she went back in time.
However, in the 3 translations of the manga I have read* the heavy implication is that Cosmos went back in time because Sailor Chaos was too powerful to defeat. I also recently read 2 fan mangas (one in English, one Japanese) which clearly depicted Cosmos returning to the future in order to fight Chaos. This leads me to believe that either Netflix's subs/dub made a mistake, or Takeuchi or whoever tweaked things.
Also, the movie is less ambigious and in a way, more optimistic than the Manga regarding Sailor Cosmos' identity and fate. The implication of the movie is very heavily that Sailor Cosmos is 100% an evolution of Neo Queen Serenity. Maybe this means she is NQS when she gets her Senshi powers back, or something else. Point is there is much less room to interpret her as say from a possible future or for her to be literally anyone else other than Usagi/NQS deeper into the future.**
Furthermore, not only does she succeed in defeating Chaos but Guardian Cosmos' dialogue also seems to be different and more optimistic. In the manga translation I have, Guardian Cosmos' last few lines have her saying Chaos is gone but could return because everything is possible in the Galaxy Cauldron. However, the movie's subs and dub actually have her say if Chaos returns she is sure Sailor Moon will defend everyone. Which sort of corroborates the earlier statement in the movie that Sailor Chaos was defeated.
Finally, the framing of Sailor Cosmos's melancholy is different. In the manga Sailor Cosmos's dialogue leads us to believe that Chaos is too powerful to be defeated, has annihilated A LOT of the galaxy and that things can never be the same. Sailor Cosmos, naturally, is left wondering what is she even fighting for.
However, the movie's presentation of events frames it more as the price of victory being so high Sailor Cosmos is left wondering whether it was worth it. Of course you could argue that her POV was the same as in the manga, the difference being that, even after defeating Chaos, there are more villains still around to fight. However, the fact that Sailor Chaos was defeated imo changes everything. Because, with Chaos defeated, it more or less means Cosmos is just kind of...tired? Plus the flashbacks (er, flashforwards I guess) seem to imply the war with Sailor Chaos was predominantly on Crystal Tokyo.
Like in the manga it was a case of she is up against an enemy that is too much for her to defeat, so it seems like there is no hope, other than the idea that everyone will endlessly go through this cycle of life/death/rebirth. However, in the movie, she kind of is just looking at things from a glass half empty POV. There is no dialogue asserting that things could never be as they were before, so she could use her OP restoration magic to just fix everything. Like, why can't she just resurrect everyone? Or go back to the Galaxy Cauldron to factory reset everything the way Eternal Sailor Moon did?
In the manga, I, and I presume most everyone else, presumed this was not an option because she couldn't defeat Sailor Chaos.
I'm not even saying this as a criticism strictly speaking, more an observation. Perhaps she forgot she could do that? Perhaps the message here is that as she grew older and more jaded she lost touch with her youthful optimism? I've always interpreted that being the message at the end of the manga in regards to the Usagi/Cosmos relationship
*2018 Kodansha, Miss Dream and Mixx. Never read the 2011 Kodansha translation
**To my knowledge, one of the musicals implies Cosmos might in fact be Chibi-Usa in the future.