I don't think anyone has recommended these female solo j-pop artists to you but I definitely highly recommend them!
Ayaka Hirahara (her voice is gorgeous, she is my favorite female solo artist as of now). I think this song is her strongest vocally, her voice is so beautiful..
Her latest album "Doki!" is one of my favorite J-pop albums.
I like her "My classics" albums a lot too (she has 3 of them). Highly recommended!
Maiko Fujita - Beautiful voice too. I'd recommend starting out with this song, it's one of my favorites
.