I see you asking why your novel doesn't get a lot of attention. As someone who has read a lot of fanfics, both popular and unpopular, perhaps I can tell you why.
The first turn off is the main character. He's pretty.. one-dimensional. I just don't feel any depth. Happy go lucky, amnesia, misses the "warmth" of a family, etc etc. If you want to write this type of character, try OC-drama, not fanfiction. He's pretty morally grey I'd say, but all of the "evil" things you say he does such as murdering and slaughtering gangs are talked about solely in past tense, we're only told he does these things and not shown him doing these things.
To me, at this point in the story at least, he just seems like a plot device instead of a character. I don't particularly know why you've decided to shift the focus of this story onto other characters, but the results show, even you yourself admitted it was a bad move making him a teacher.
In my opinion, you shouldn't have made him OP. A more ideal scenario is him knowing a little about his powers, but him having to train in order to unlock them. That could have been interesting as we could actually see him struggle and grow, but if you really think about these past 70 or so chapters, nothing really happened. I mean, there was a dream (flashback?) of some sort and he became a bit more mature and silent but nothing else really happened towards him. He's just incredibly bland. I get it already. He isn't a hero, he's just doing it for "fun". No need to say it 30 times. If you hate the prospect of him being a hero so much, just make him a villain.
Another thing is the.. err. romance? It was very quick and forced. I think there could have been a more dramatic encounter, and I feel as if Eri opened up to soon. It should have taken along time for them to trust each other, but I honestly cannot be bothered to stick around to see their relationship anymore. You already said you wouldn't be writing any R-18 because you're not 18 (so do you not watch or read hentai because you aren't 18? there's a lot of things you aren't supposed to do until your 18 but you probably do them so I see no problem with writing fictional lemon but you do you I guess).
Eri has pretty much been reduced to a background character who the MC goes shopping with sometimes or something. Should have focused more on them instead of the main cast of BNHA.
Furthermore, I'm really not a fan of child protagonists. You keep hyping up his "mysterious" identity, but it's getting tedious at this point. It just felt like nothing really happened at all in the entirety of the story. It just feels forced, bland, and congested. I think you should try writing OC instead of fanfiction, you have a very hard time developing the main characters of your own story and instead opt to develop the mcs of the anime your mc is in. Not my thing.
