@Varae
Nice find, I recognized the artstyle but you pinpointed it first. mangadex link for h-prequel: https://mangadex.org/chapter/727795/1
@tranquilsea
yes, at least one of them is his pen name (maybe both) : https://e-hentai.org/g/1515371/e17572f330/?report=select
https://mangadex.org/search?author=Mikuni%20Mizuki
@Roiisu
Yes, 芽衣 (the girl- I checked her name in both h-prequel and this manga's raws) just changed her hairstyle slightly, tomohisa looks the same.
I checked author's Twitter where the raw is from, and they listed their name as: 四葉真/みくに瑞貴@色々準備中
四葉真 = Yotsuba Shin Makoto
みくに瑞貴 = Mikuni Mizuki
I have no idea who Yotsuba Shin is, but The author is definitely not "Yotsuba Mikuni", So I changed it to Mikuni Mizuki, considering this is related to a manga that is credited to a Mikuni Mizuki.
EDIT: Shin Makoto. Author has 2 pen name, Mikuni Mizuki for eromanga, Yotsuba Makoto for mainstream manga. Grancrest Senki manga is drawn by him.