Here is one idea - pretty standard hedge plant.

Ask your family what they would like to see, not designwise, but plant materialwise - color, texture, fragrance. Even ask your mother-in-law. You have provided some architect's dream, with the bold geometric forms, the bold palms and cycads. How about something warm and fuzzy. Something to rub your hands on as you head to the door. Some color and fragrance beyond the white and palm trunks. How about vines to soften the hard edges of the planters, fill them with ground cover plants so you won't see the mulch in a couple years, add bulbs of all seasons to bring cheer, glossy leaves to brighten your day. Bring nature to your entrance area to warm up the arrival of your mother-in-law. How about saving room for a couple chairs and small table, to sit out and prove that people actually live in this piece of marvelous sculpture, and add the warmth of an outdoor room. Spend some time at the library with books about the architecture of Frank Lloyd Wright. He is the one who introduced the architectural design idea of the outdoor room. Come to think of it, I don't remember ever seeing a garage door in any of the photos. Guess I'll have to take another look.

Thank you all for your comments. Well, it's been a few months since my original post and we have since finished putting up the retaining wall and planting the hedge. We decided to go ahead with Murrayas - planted about 34-40 cm apart. I gave them a good trim about a week after planting at the end of Autumn and another quick trim at the start of Spring. They have really filled out in warm weather and I think the gaps between individual plants should close by the start of winter. I'm really happy with the choice - the hedge looks glossy and lush and the sweet smell of Murraya flowers is just beautiful. We didn't do any underplanting, but I'm still thinking about putting in the dwarf agapanthus for the inside border. A couple of weeks ago we planted some Rhaphiolepis "Snow Maiden" and Rhoeo along one side of the retaining wall to provide a border for a footpath and I'm really happy how it turned out.

Congrats on choosing blocks for your garden bed. If you're looking for trees, I suggest considering a maple tree. They grow nicely over time, providing shade and privacy. I recently moved to a new house myself, and I'm also exploring plant options. Back in my hometown, we had a reliable tree service that helped us with planting and maintenance. Maybe you can find something similar in your area. I came across this website, https://www.deepdale-trees.co.uk , which might be helpful for you. They offer a variety of trees and services.