What to plant to fill the gaps? Brisbane Based

I am slowly getting into gardening and I was working on a blank canvas other than the tree (might change at some point). I used the attached photo as a reference point but added a few extra plants that I like (silver spoons, chalk sticks and a few black rose succulents).

Things obviously need to grow and spread out more but I am a little unsure what to plant in these highlighted red gaps to make things harmonious.

The gap on the left is quite large and only receives decent sun in summer but I do have a small white Gaura Lindheimeri there (been cut down for winter) so am tempted to bulk that out to fill more space and then it is similar to the reference picture.

Would love some opinions on what others would do to fill these spaces.

Thanks in advance. 🖤