Herniaria Glabra has a slow growth rate so consider buying more plants. Sun exposure? Looks like the area is in mostly shade which won't help the plant thrive. What zone do you live in? Usually masonry installers don't take the time to replace the soil between paver with loam. You may need to inspect the soil and amend the area to a depth that is sufficient for the plants roots.

Yes, Mina - rupturewort is very responsive to being cut into smaller chunks :-) You should be able to get 4 plugs from a 4" pot. But I agree you will need more than a single flat. I spec it a lot for my clients - it grows very nicely here.

ps. did you decide to go with the Azara? Did you find some?