Do you have a view? If not add height with some trees. I’d work with Tasmania’s natural beauty and go with natives. Gardening with Angus has a great plant selector on his site. You could go lush green and sophisticated with mass plantings of different size Acacia (Limelight, Fettuccine and Cousin it) and under planted with Lomandra (Verday or similar).

I think you will have a difficult time achieving a tropical look in Tasmania, and at the same time missing out on one of the best things about living there. The temperate climate. You have access to garden species that those of us in the warmer states could only dream about. Why not use that?

I'm not convinced you need a retaining wall anywhere except possibly under the patio.

You definately need some bushes between 1-2m in height in that empty garden bed near the port box, to add a bit of visual interest and take away the fisheye effect of the sloping land. You are definately going to need either deciduous or frost hardy species.

Off the top of my head these would do well there....

Silver birch, weeping or upright. It's a beautiful tree that loves the cold weather and gives you the added bonus of as much natural sunlight to your home as possible during winter when you need it. Also lovely soft screening that won't become a jungle in the front of your home. Pairs beautifully with Japanese maples for a lovely woodland look.

Rhododendrons make a wonderful feature plant with it's spectacular display of flowers. If one of these is too large look to azalea's which are just low growing relatives.

Under your deck seems like the perfect location for a Tassie native, the Dicksonia. Come on how could you not grow these?