The depth is quite narrow so, it's best to keep with small plants. You may need to stake plants to stop them from flopping into the lawn. Geraniums, Violas, Heucheras are good plants that don't flop too badly. You have planted a lot of plants already. Whilst they look a bit sparse now, in a year or two, they will all get wider and taller. To create impact, you may need to either create big clumps or repeat-plant all the way round. If not, it is difficult to create height and good contrast between plants. Also look into tall airy plants that can add some movement and height against lower growing plants. Verbena Bonariensis and Knautia Macedonica are two examples of plants needing very little base space, but can add some colour and height.