Even if it was a band, at some point Izumi Sakai, the only member of Zard to be part of the band during all of its duration, was recognized as either by her name or the band's name. 10 million sellers including two 2x and two 3x. 8th best selling music act in Japan.
Matsutoya Yumi is around since the 70's, but she had 9 million sellers on the time span you picked, including 3 2x million sellers and a 3x million seller. I don't know if she counts. She is the 6th best selling act in Japan. Her comercial and popularity peak was during the 90's, including her best selling album and her (probably) signature track, Haru yo, koi.
And just because you put YUKI
KEIKO, the lead of GLOBE, one of biggest japanese bands of the 90's with the 7th best selling album of Japan and a total of 5 million sellers (one 5x, one 3x and two 2x).
Miwa Yoshida, vocalist of Dreams Come True, the 5th best selling music act in Japan, around since 1989. 10 million sellers during that time span, inclusing a 3x and two 2x.
I have the impression KEIKO is seen as a name of her own (like Gwen Stefani or something) while Miwa is an integral member of DCT to the public.