please help with what sort of plants to plant in this garden to fill it out and make it look established.
Where are you located and more information about sun and rainfall would be helpful
Climate, soil, etc? No point suggesting plants which won't do well in the conditions.
Are you sure the existing plants won't fill in the space? I can't quite make them out but the row of sprouty looking things appear as if they might grow larger, and a number of plants look clipped, and the tree looks fairly new. It's a fine line between quick effect and overcrowding. Sometimes watching the tv shows which make it look easy I recognise the plants they are using and think that within a year or two the owners will be pruning them weekly or pulling half of them out.
Balcony wall garden plant suggestions
Garden Planting Help
Please help! Trying to fix up what was once a thriving garden!
Desperate for advice on plants & landscaping for our brand new garden
Illawarra area, humid and coastal conditions
Coastal enough that salt spray is an issue, or merely that happy zone where you don't need to worry about frost? My eye is still drawn to that row of plants which look as if they will in time fill the space better. But a nice lush ground cover could grow around them I suppose. Convolvulus? Star jasmine, as long as you don't mind the fragrance which seems a love it or hate it thing? There are probably plenty of others I'm not familiar with as I live in a frost zone.
Look for plants that thrive in your area, add structural plants to add interest, silver foliage looks great , gravel or stone mulch adds to the look.
Look at the Instagram account for Adore Magazine. she has a section on her garden which is coastal and it looks good