https://idol.ne.jp/ is a website containing discography lists for Japanese idol singers and groups active in the 1970s and 1980s (and some from the early 1990s). I've been using it as a reference for years.
Example artist link: https://idol.ne.jp/hagayui (for https://musicbrainz.org/artist/d2f66b8b-7cd3-41fd-a5eb-6586ece97490)