Companion suggestion for buttonbush?

I'm in central md, piedmonts.

I have a medium sized bed, right up against the bricks, but this is the garage so it is unhealed.

I put in 4 buttonbush last year and they're doing great, but I want to fill in around them.
The area is sunny all morning and full shade in the afternoon due to the house. It is pretty wet most of the time.

I'm considering bee balm or mistflower, but would love suggestions, especially for stuff I may not have already.

(P.s. I transplanted in some violets, which look burnt right now, but they'll come back next year as groundcover)