After several months I've eventually finished this game. I've played other 'mixed-genre' visual novels before (Danganronpa, Persona and Ace Attorney for example) and generally enjoyed them. And yes, I did find 13 Sentinels interesting and fun to play for the most part. I'd definitely recommend trying it.
However I think the way the story was presented made an already complex story unnecessarily confusing at points. For context, the story is told from the perspectives of 13 separate protagonists, in short chunks. Because of the way you unlock different sections of the story (either by getting to a certain part with one character, or making progress in the battle mode), you'll be shifting through each of the individual stories constantly.
In some ways this is a neat way to tell the story and keep things mysterious, but when there are so many characters with branching storylines it becomes a lot! Granted, it probably didn't help that it took me several months to get through it - that was mainly the result of the battle mode feeling like a chore to play at points.
Despite all this I did really like where the story went, and it mostly makes sense after one playthrough.
Some aspects of the story that I'm still unclear on:
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The whole deal with the multiple versions of Morimura/Iora/Chihiro. Morimura was a previous version of Iora from a previous loop? And she wanted to implant her memories onto Iota?
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Similarly, Ida's story. Amiguchi is another version of him?
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Why was Chihiro trying to force another loop? She wanted to be the one in control somehow? And similarly, why was this the final possible loop?
If you like visual novels, confusing anime storylines and science fiction you'll probably enjoy this game. Visually it's very impressive with some beautiful animation on the characters and painterly backgrounds. The voice acting and music is excellent, perfecting fitting with the tone of the game. It's likely not for everyone, but if you're willing to accept its shortcomings it is worth playing for the fascinating story alone.