This story line does exist but wasn’t exactly romance but more like a AFO using him and Izuku just going with the flow since he had nothing better to do.
It was also an oneshot which was just not continued to give the characters depth. And I don’t think you should worry too much about copying story lines, considering how big the anime community is, pretty much all plots have been used atleast once and everyone takes inspiration from eachother, all is fair except copying word for word.
What I personally loved about the story was that AFO took care of Izuku and made sure he was comfortable despite it being very obvious that he was eventually going to use him, so that eventually Izuku himself didn’t want to leave. There was very little actual romance in the og oneshot.
Tartarus was very vaguely described there, it was more like a breakout and everyone was trying to get out, while Izuku was battling with his inner conscience on whether he should leave or stay. Izuku being a former hero had alot of people in there who would kill him, so he and Lady Nagant, leave together, when they got separated he started going into heat, when he meets AFO, who gives him the offer. It was essentially a life and death set up, since if he stayed he would get forced upon by some villain who has a vendetta against him or he could go to AFO and well- live.
And I absolutely adore your stories and how even the villains have depth and aren’t just straight up evil-
Can’t wait to read any future stories!!