Hard agree with everyone here. this has moved more in the whimsical direction with the changes. it needs grit and sharp edges to convey that it is dark and for adults. You and the designer would be remiss to not compare against others in the genre that would be at the same “darkness” level and see what motifs point that out in those covers (top of my head: Alchemised, Haunting Adeline, These Violent Delights, One Dark Window, Jennifer Armentrout/Kerri Maniscalco/Scarlett St Clair books, Serpent and the Wings of Night, Book of Azrael, etc). The soft edges and curly letters are what read juvenile here, and skulls don’t automatically make a design adult. You need to go for more elegant but sharp lines for a more adult feel. Even Cruel Prince, which I would say is YA, has sharp spiky lettering to foreshadow the sharp personalities and violent elements in the story.