I remember I read part of this shoujo manga a long time ago, but I didn't save it. It was about a high school girl who helped an elder man, who turned out to be the leader of a gang with three sons. So he said that whoever she married would be the next leader. Ohh, and she randomly has a stubborn streak which makes her act like a wife of a gang leader, because she admired them.
The never-ending collection of stories I've personally enjoyed about an OP MC in a variety of different situations. Usually an isekai or reincarnation.
Tends to avoid harem (or blatant romance) but there may be a few. Trigger warnings will be added. A blatantly self-indulgent list in no particular order.
Part One is here: https://www.mangago.me/home/mangalist/1132346/