On the top level you could grow both edible ginger and turmeric. Growing these plants will allow you to enjoy their flavours in your food from your own garden. They would also give you a background level for the whole garden. Some of the cherry tomatoes would also give you a ground cover and plenty of fruit on another tier. Checkout the lovely grevillea ground covers on the net. The bottom tier would be soft with some of the grevilleas available.

They have shallow root system- so near your colourbond fence- the heat in Summer will not only dry the roots, but also burn them and the foliage. They require regular mulching and fertiliser, else the leaves yellow and they start dropping. Given you want them as the 2nd layer in your already narrow hedge, it will be a full-time job keeping them happy I think. If you are not a fan of this level of garden commitment- check out the regular mondo grass (not mini). It grows like a weed just about anywhere! Aganpanthus is another one. Both would be ideal 2nd layers and maintenance free.

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.

Your site is far too exposed for Maples. They are small trees that really need to be buffered from winds and too much sun by other plantings. They also need the soil constantly moist which will be nearly impossible on that slope. You really don't want to grow magnolias anywhere near the footings of a house. Have you seen how large a tree they are? Since you are in a temperate zone and like autumn foliage I can recommend a gingko tree. It will be right sized for your yard and give you that similar lighter than air look and autumn colour as maples but it's more hardy. Another option are silver birches. I would hedge the boundary for privacy and also to cut down winds to your garden which will dry out the grass and scorch it. With that slope you are going to need more rocks I think to prevent your topsoil from washing away in the first rain. It's really no use of us to provide you a list of random plants. What you need to do is put together a series of photo's of gardens you like the look of, then find plants to match that style. If you don't plan gardens then you end up with the ubiquitous suburban patch of grass and odd bushes look instead of a lovely garden.