I wanted to provide an update as my mailbox garden has started to fill in nicely.
It turns out the ajuga did come back, but reverted to the dark leaves. The poor sedum I planted got taken over so I moved it to a safer spot this spring.
I also lost the agastache I'd planted in place of the guara... I'm beginning to think the spot is cursed. I actually think it may just be too moist so I've planted cardinal plant (lebelia cardinalis) in the hopes that it will appreciate the moisture and thrive in this spot. In the photo you'll see it's the bare spot just behind/left of the rock.
My favorite thing about this garden is that everything takes turns blooming... first the daffodils, next clematis, then the weigela, next the pensetmon, hopefully then the lobelia, coreopsis, and cone flowers. This variety of cone flower is just the cheeriest! It's a double pink variety and seems to bloom forever! Then finally the aster blooms and the northern sea oats put out their seeds. I've just been really pleased with this garden!
Also, this clematis is so stunning and visible from very far away, which was my goal.
What started out as such a hodge podge of random sale plants has become one of my favorite beds. When neighbors ask which house is ours they always say, "Oh you're the mailbox house!" The wheels have tiny solar string lights on them and look nice at night, illuminating the lavender blooms of the clematis.