, I have not grown dianthus in years, but your plant looks like all those mushy blooms need to be cut off to promote new growth. Hopefully, someone who grows dianthus will chime in, but I can't think of anything else to do.
I just planted 24 marigolds and even though they were in bloom, I cut off the blooms so the roots had a chance to develop. I did not want the blooms to go to seed and take energy away from the rest of the plant.
Edited to add a welcome to NGA and the forums!