True, any recommendations of stories that go into that direction?
Unfortunately there aren't a lot. I can think of only five that meet the transformation criteria at all.
Mirai Nikki and School Days (though sitting through School Days is a special kind of unpleasant, to watch such quality girls lose it over such a twat waffle, actual middle school was less unpleasant) are the og of course. Happy Sugar Life (from my avatar) is a rare mutual pairing for a truly extreme yandere. Re:Birth: The Lunatic Taker is a nice combat power oriented pairing though it's a sisxbro pair and there's no satisfying resolution. Koharu no Hibi - fl presents as a complete yandere from the start, ml is into it, she knows his every weakness and plays him like a fiddle and he - ain't mad at it. Finally
You, featuring a rare live action male yandere and it's low key a complete gem, ml transforms perfectly even fulfills the muse role helping their partner become their ideal selves as well, did not expect season 2 to be as good as season 1 but it managed I'm on season 3 now there are 5 seasons to binge on Netflix!
Dishonorable mention goes to Ijousha no Ai, it's a transformation tale but it's a sadodere transforming everyone around her into her own hellish ideal. Fl often gets mistaken as a yandere but there's zero interest in actually meeting anyone's desires but their own. Great carnage, zero actual care for her target or victims. Honestly there could be dozens in the dishonorable pile, every time a romantic partner goes a little crazy or murderous they call 'em yandere but for me they just don't qualify.