I would plant along the fence line trees like Magnolia (Little Gem) or Hibiscus, and there where the wider part - low shrubs under the trees or stripy Dianella. All these plants and trees are hardy and easy to take care of. Trees don't drop the leaves and slow growing. Otherwise you can place pots with bright plants (Frangipani grows well in the pots). You can mulch the soil under the trees or put some pebbles or gravel like I did (see the photos).

Strong geometry, cool blue and glass and a plain lawn background. to fill the diamond with bush is going to cancel out its strength. But something different to the slightly sterile design compilation … if the finance is there, get two or three massive raw rock slabs and one spectacular architectural tree. resist going vertical .. layer them horizontally to slightly overlap or break the hard lines of the diamond, but not to obscure it. Then choose you vertical with a sculptural tree. There are vertical cherry trees with spectacular autumn colour which would contrast with the rocks (texturally), and be floaty like the water in the pool…. whatever the tree, lift the foliage up from the soil level, and underplant with jut one kind of tight low ground cover, even moss (if your climate can support) or ground hugging juniper that you can train inside the diamond. This combination of elements is rich for the eye, softens the geometry, and adds interest which is lacking right now. The place for shrubs and grasses and palms is to border the lawn.

Some great planting. First go to a local nursery with the photographs and ask for some recommendations. Buy some small stuff from them Search your local community page for plant sales for some large hero plants, especially ones that will give some colour and interest. If natives are appreciated, there will be a local native community group who could provide great information and plants. Best of luck

Love legendaryflame ‘s idea of a spiral, you could create a stylish look by planting each herb in its own formal area, herbs like parsley, mint, sage, rosemary, oregano, thyme, basil, edible flowers like nasturtium could look great