ITA with Tartan ... groundcovers ... many of them will flower as well, though most of that would be in spring.
Add to the list ... periwinkle, sedge,
Being that it's a Sundial Garden and you have numbers placed, I'm thinking you probably need to consider the height of the plants, so they do not cover your numbers.
You mentioned standing water after a huge rain ... of course, we have no idea if this is common in your area? or only once a season. That needs to be taken into account as to which plants would survive flooding! Likewise, during dry periods, will you water or do they need be dought tolerant?
The possibilities are endless ... and this could be a long term acquisition project, to enjoy for years to come.