Yes, I wouldn’t call some of the above-mentioned underrated. People that come to mind the OP might like:
Agnes Obel—classically trained singer/songwriter from Denmark originally, listen to her first album Philharmonics (2010) and go from there. Aventine (2013) is my favourite, very beautiful and serene.
Beth Orton—a folktronica singer/singwriter, from UK, a long career but try her latest Weather Alive (2022). Maybe a stretch to call her underrated but depends where you live.
Martina Topley-Bird—also UK best known for her collaboration with Tricky (likely not your taste) but solo work spans Quixotic (2003) through to Forever I Wait (2021).
Less likely for you perhaps but very good if your taste skews more rock-adjacent:
Erika M Anderson (aka EMA)—from US, best known for Past Life Matyred Saints (2011) but recent Exile in the Outer Ring and Outtakes from Exile (2027 and 2018 respectively) are also good, as is her moody soundtrack for Tara Subkoff’s movie #Horror
Kelsey Lu—also classically trained, from US, great singer/cellist, try their debut album Church (2016) or Blood (2019) which is impressive: the track Foreign Car is much more fun than audiophile staple Fast Car. Wonderful voice in the lower range too.
Mariah Rose Piera (aka Poppy)—from US, a genre-fluid corpus but her latest Flux (2021) is fabulous and includes the first performance by a female artist nominated for Grammy in the metal category (I did say rock-adjacent). It really took that long, can you believe it?
And some electronic/experimental flavour if you want to stretch:
Tahlia Barnett (FKA twigs)—from UK, recent album Magdelene (2019) displays her very nice vocal range and compositional talent and is fairly accessible (also an excellent dancer/choreographer). I guess ditto on the under-rated thing.
Nat Ćmeil (aka yuele)—originally from Singapore, their first album Serotonin II (2019) is fairly accessible, but the more complex Glitch Princess (2022) is the best album anyone has produced this year (that said, you’ll likely hate it).
Some other female performers I’ve come across (more or less) recently that are less known than they should be (some new, some not so, a mix of genres with sonically interesting but not necessarily mainstream vocalisations): Alice Glass (escapee from Crystal Castles), Cherry Glazerr, COUCOU CHLOE, Ethel Cain, Fifi Rong, Hania Rani, Honey Gentry, Jack Off Jill, Julia Holter, Kilo Kish, Let’s Eat Grandma, Lil’ Bo Weep, Midwife, Nastya Kreslina (half of IC3SPEAK), Pan Daijing, Reba Fay (aka Swan Meat), REI AMI, Rosie Diamond, Shygirl, Stella Donelly, Utaha (new singer for Wednesday Campanella), Yara Lapidus ...
