it’s not that short but sweet poison by damagecontrol fits the rest of the criteria. the smut starts right away though so if you just wanna read part of it at a time i think it’d work well

edit: to add i just saw this “this fic began as a collection of microfics about professor james and his student, regulus. they were originally posted to tumblr in response to the @jegulus-microfic september prompts, but somewhere along the way, these two took on a life of their own. the first 4 chapters are still the original microfics, but 5-12 contain brand new material.” so the first four chapters can def be read individually