^If you don't like her childish vocal, then I'm confused what song should I recommend lol she sang like that generally, it's more like she had songs that sounds childish like Ramen Sanpun Cooking or Kataomoi Dial but those are fun xD
I tried to think of some of her songs that wouldn't sound childish or sort, these are some of that I like
Mostly are ballads or mid tempo, her cheerful songs might sounds a bit childish for you maybe