Some of your plants are already big enough to divide and others will be by autumn. No need to spend more money if you don't want to. I rarely buy plants but when I do I try to pick ones which can be divided immediately, so I get two for one. Is Ph Phlox? They could be made into 4 or 6 plants right now.
Agree on making the circle circular, although personally I'm not a fan of small island beds. However, if you plant some things in the island and repeat them on the other side of the grass in the large bed, you could get the effect of a meandering path through a larger planting, rather than an isolated, unanchored bed floating in the lawn.
Q