What happens with every series when the Author is like "I'll explain it down the road"
Then we get to the destination and the Author is screaming "FUUUUUUUUUUUCK"
That's about where we're at.
Where did it go wrong? Well it started off pretty fucking good. I liked the characters, I liked their motivations, and that there were multiple perspectives for what is essentially another series in the death game trope. However the trope tag is what encompassed the entire genre. The Author started to become scared to take risks and it showed by characters becoming able to avoid death magically and nonsensical brain 'upgrades' to allow people to fight at blinding speeds with super strength. The question you need to ask yourself is if we needed any of it. See this is the problem in writing Fantasy. It's so easy to be like "fuck it super strength" and harder to say "how would this character escape this situation without it". Which is ironic because that was the purpose of the entire start of the manga. Our FMC was by no means able to compete with plenty of characters in the series. And that's just the thing about this series, it's so boring when we can't tell how strong someone is. You spend so long trying to get adjusted to someone's power level and then you turn around and fuck it by giving them brain hacks. Then we don't know how strong anyone is. That coupled with fights becoming less realistic as time went on, it's like the star wars problem with the Phantom Menace. Everyone is doing twirls and shit and the realist is sitting there like "fucking stab him. the second he spins. right there. he's not even looking at you. just stab him."
If you're so powerful that you can do all this crazy shit destroying the environment and everything just blink over and pop someones' head like a watermelon. Quit fucking around.