Singers:
Amanda Somerville (Trillum, Exit Eden, Kiske/Somerville)
Marcela Bovio (Stream of Passion, Mayan, Dark horse/White horse)
Bands:
Kobra and the Lotus - Heavy Rock
Stream of Passion - Progressive Metal
Tardigrade Inferno - Cabaret Metal
Volturian - Symphonic/Pop Metal
I added a folk metal ballad, and almost half of the recommendations were the complete opposite (fast songs with growls), no matter what options I selected.
Anyway, I liked it. It might need some refinement and may not work for creating a playlist (at least for me), but it helped me discover some tracks that are exactly what I was looking for. Thanks!