Carpet suggests you want year round cover - both Astrantia and Brunnera are herbaceous perennials and disappear in winter. Other options include Ajuga reptans, Campanula muralis, Heuchera

I'd alter 'Heuchera' to 'Heucherella' and/or Tiarella, if the Brunneras are in shade. I find that Heucheras don't like too much shade - except H. 'Dales Strain'.

I have a "carpet" around my Brunnera of a low-growing, spreading Sedum. Sorry, I don't know which one, but perhaps a garden centre/nursery could help. There are also a few Cyclamen peeping through, but of course they are very slow to spread.

There is a rather nice variety of Pulmonaria called Diana Clare that has silvery green leaves will compliment brunnera leaf colour , is perennial and will tolerate shade.

You ll obviously need a few to spread but should self seed readily.