I like your daughters name, I think this is the first time I have seen Ashby, or at least took notice.
I am not a fan of Mercy or Mabel, so that may be the reason why I do not like it with Ashby.
However, I like your choices of Harper and Matilda.
Not great with combos, so it will just do a few:
Piper Rose (Well, maybe not, just remembered the Charmed, association. Unless that does not bother you.)
Marley Rose (I met a beautiful little girl with this name (spelled differently). She was so happy to introduce herself, because she loved it so much.)
Other names you may or may not like, but may be able to inspire you: