It does not matter if the mc is girl or boy but ABSOLUTELY NO HAREM
If I had a nickel for every time someone asked for this trope I'd have two nickels, which isn't a lot but its weird that it happened twice, right?
Manga Rec:
Please Bully Me Miss Villainess!
https://www.mangago.zone/read-manga/please_bully_me_miss_villainess/
It is a GL, and the main plot is not about her becoming a kid again but it does happen.
Thanks
The manga is not on this site, but the anime is currently airing. I'm sure you can find the manga on another site
Isekai de Mofumofu Nadenade Suru Tame ni Ganbattemasu
I want to read where an adult reincarnated into a kid
NOs:
NO LOVE INTEREST WITH HUGE AGE GAP OR BASICALLY JUST NO LOVE INTEREST
FATHER AND ENTIRE FAM IS NOT AN ASSHOE
FANTASY