I don't think the mutilation is extra - it's something that represents physical proof of their bond, that they're exclusive to each other, and probably why Mizuki can accept that Sayori won't tire of her even when they are apart, like what happened to the couple in their novel.

Much more romantic (and painful) than rings!

What toxic Yuri does to the human mind.

But also don't get me wrong I'm not mad about any of this lmao. Toxic co-dependency go brrr.

To me there seems to be a lot of implication from the senpai's (even though appearantly they freaky af) that all of the hurt they inflicted on each other could have been avoided if they just were honest. Like they could have arrived at the point they are at now at without mutilation if Sayori said "hey I really want to go to a different school but I love you so we'll figure it out" lmao. Obviously mutilation via box cutter is an extreme example but the mangaka seems to love their melodrama. Im certainly not complaining lol.

Im sorry I can't get over this girl having a full weird reverse flash tier villain monologue about her feelings every other chapter its too funny. And Idk about anyone else but im definitely dropping if the the plot doesnt get moving soon. Five chapters in and it feels to me like there's basically no momentum.

Also there is 0 chance this is AI. The artist has been posting art on pixiv since at least 2020. These consistently weird proportions are quite literally evidence of the human touch. Probably from long developed bad habits if I had to guess. Not enough referencing or proper construction, maybe.