You could lay out a garden hose and eyeball it first. Be careful not to make your curves too sharp. Mow along there with your lawn mower and don’t make the bed curvier than it can easily mow closely.

The way to split your grasses is with a spade. If that doesn't work, try a saw. A lilac needs some distance from the grasses. You probably want some plant that is in between the size of your large grasses and the sedum. Veronicastrum Virginicum is a nice option.

We called this a serpentine garden. Created using landscape retaining wall block.

that looks beautiful.

@ibooras54 - FYI, here is an expanded view of the whole setting including a walk along the serpentine.

https://youtu.be/yh0ZL-PwsMQ?si=Lk1LXm6zyAOMcjVt

The grasses look like they might be planted too close to the lattice.

Using elements you listed: rocks, lilac, divided grasses, sedums (tall like Autumn Joy assumed, not ground cover like Angelina), Stella d'Oro daylilies. Added some red dianthus to echo the red umbrella, white foxgloves for height and verticality, dark ground cover (chocolate chip ajuga?) to exclude weeds. All are relatively low maintenance, although the biennial foxgloves will seed around. I enjoy the surprises that creates, but if you don't, there are perennial foxgloves now (less vigorous, though, in my limited experience). Ideas, anyway . . .