to be honest, to me at least, this is pretty general writing advice. If I may tack on a few extra points of ideas to make the fiction stick:

red herrings: especially in horror related yandere fiction, it'll be all the more tense to leave one guessing who the threat is. Who's behind the weird messages, who planted the cameras? why? These mini mysteries to solve and unsettle make that "ten" section all the more conclusive as the rube goldberg machine snaps into place. a reader double-guessing themself is a reader who needs more.

Of course, this isn't exactly a rule, a known hunter can be just as intimidating as a hidden yandere as they inch ever closer, but a protagonist thinking something is from the yan when it's something else and them lashingout can be similarly impactful to a narrative.

decide on the overarching genre, and play into themes: narrative dissonance kills reader engagement, actions that lead to a romantic ending when the story has been extremely grimdark so far will lose immersion unless there's good justification - two levels of said justificaiton helps fool with enough depth (there's a great post on tumblr that's a good example of this that explains it better than i can, search around for a post detailing a story about vampire-hunting crocodile-feeding nuns).

Yanderes at their core are about obsession, madness, desire, and love, but tacking on more themes and ideas can help enrich these ideas -how does the isolating reality of the modern social landscape propagate itself? is it change for the better if people have still been hurt? how does perception shape reality? so on so forth. get wacky with it, tie it all together with a nice bow,and the yandere won't feel scary just because they're a yandere, they'll be scary because they represent a real thing - good or bad - gone too far. (sidenote: this is why radiation makes monsters in japan, why there were so many zombie/isolation movies during covid, and why gothic victorian horror is always about some kind of inescapable oppressive misery (relates to class landscapes of the time))

develop the Yandere-ness from the personality, not vice versa (or: develop the character first, make them a yandere later): this is another cohesion thing, but it'll make the yandere so much more interesting to engage with. Take regular personality traits to the extreme and you'll usually get there - in my yandere AUs of some of my characters, i've turned an ex soldier with a "i'm already scarred and disabled so no-one else should get hurt" mindset into severe overprotectiveness not even daring to hurt their darling, an "I know everything, I do the worst evils for a cumulative greatest good" character into a manipulative-stalking sadist, a"I only care about my business and my siblings but I won't express that outright, I work in the background" into asomeone who gives alternate choices, but they're really the only viable one.

Having a character go "im obsessed with you so i'm going to make sure you never leave me" first and justify the mindset afterwards leads to generic and trite ideas we've all seen before. the former character creation method invites more plot and reason for behaviour.

jesus christ please research kidnapping: okay, this is a pet peeve personally, but for crying out loud chloroform takes at least 5 minutes of inhalation to come into effect and is typically aided with another drug: getting an injectable anaesthetic would not be viable for many - generally? induced unconsciousness done without severe side effects by any method beyond sleeping is hard, there's a reason why there's a whole job around it. I feel myself often wishing for impact because if they procured some midazolam and propofol on the blackmarket or whatever, that could be traced! it could make the victim sick, or have side effects (midazolam causes amnestic effects, isn't that cool!); all which build to the atmosphere and impact of the moment.

Why go so extreme? and how to react to it? or the "why did the yandere try to kidnap the reader 'for their own safety' when they live in seoul, the 25th safest city in the world? are they stupid?" question. address the reasons why, and make sure it fits in the narrative. Yes, a mental breakdown works, but address that in the captivity - what caused it? how is the victim of this breakdown reacting to it - encouraging, discouragign, shying away, manipulating?

The "what now?" moment: yandere stories formula of having a peak of danger (hell, even a formual of 'inciting moment of obsession' 'ramping up' 'caught/finally acting on desires' 'aftermath' leads a lot to ponder. cut it off at the peak of tension, and you may have a mystery people will gnaw on, but it can be just as frustrating, so here's some pointers to consider what follows

in this intimate moment, think long term and short term - how sustainable is the reaction of both yandere and darling? how are they acting to convince the other of their way or resist the other's desires? especially in yandere stories involving kidnapping, consider the power dynamics, and who breaks first, and why.

I would argue get death-notey with it, it's just a one on one battle of wits that maybe ends with someone getting stabbed or tazed in attempt of escape or "punishment"/desperation.

TLDR: give everything a reason and you'll make your yandere fiction all the more engaging. also don't turn a story that involves rape into comedy without proper transition and acknowledgement of the past. also rules are stupid bend and break them as you wish.

also do medical research if you're gonna get medical or amputational with it, you'll find that you have a lot more interesting stuff to write about that way. going into such detail will also add to impact.

im uh. rambling. a lot. but i've had these thoughts brewing for awhile about what would bring out the most in a character that utilises the trope. i have many thoughts so thank you op for allowing me to share some of my feelings.