Knowing your climate and the soil and orientation of the site is useful to ensure viable suggestions.

Yes that might be helpful...
I live in Perth. The area get some morning- midday sun. It's quite shaded in the afternoon. The house faces west but we are on a narrow lot and tge back doesn't getvthe harsh afternoon sun.

That sounds good. I've found even many plants labelled for sunny positions appreciate some protection from the afternoon sun. Perth has sandy soil doesn't it? I have heavy clay so the lower part looked potentially difficult, which is why I asked.

The retaining wall looks nice, so while some cascading plants will enhance it I don't think you want anything that will grow too rampantly and cover it up.

How do you feel about perfume? One beautiful plant that works as both a ground cover and a climber is star jasmine, but it has a rather divisive perfume which some people love and some people hate.

For ground cover/climber Hardenbergia also comes to mind.

I'm most familiar with plants that tolerate clay and frost though, so my suggestions are limited.

Thank you for your input. Yes we have sandy, soft soil. What you see there is landscape soil mix, hence the colour.
I don't mind Jasmine, however hubby doesn't like it 😏
I will look up Hardenbergia as I'm not familiar with it.

Thanks Julie, I love those ideas! How do you think bamboo would look at the back as a screen plant? Also, do you think a strip of lawn and maybe stepping stones on the bottom or stick with plants?

Bamboo is a great screen but they do shed a lot of leaves and it can be a bit messy, a row of ornamental pears would look beautiful in all seasons, dichondra will be fabulous trailing over the retaining wall, stepping stones would really set it off, you could incorporate them into the lawn, you could always include a bench seat as well

Thanks Julie, love the ornamental pears but I think they might be too tall for the area? The stepping stones look fabulous. I definitely want to incorporate them into the design.

If you get bamboo make sure it's a clumping variety not a running variety, and check the diameter, some get quite wide. I think so close to a boundary fence I'd install a root barrier to be sure.

I love the dichondra idea. The one in the pictures is 'Silver Falls' isn't it? (You always find the most beautiful inspiring photos Julie.)

What about cordylines for the "lush, tropical vibe"? Where I live we can only grow the tough cabbage tree variety but I think you can grow the smaller tropical looking ones?

Yes I will definitely look into the clumping variety. I'm still wary about bamboo though even though I like the look of it.
Still confused about which way to go for the screen plant. Problem is, is that I need height but not too high, if that makes sense! I'll just need to keep looking.
A few cordylines might be good too. Thanks again

Have a look at Fleming’s website for inspiration for trees to suit, do palms do well in your area, if so you could try golden cane or rhapis underplanted with Xanadu this would give a tropical feel, yes dichondra silver falls is just beautiful. You could add this as well to spill over in a tropical garden.

None of the suggestions so far, hit your brief which was for a lush tropical vibe. Nearly all the plants suggested are for temperate gardens. To create the tropical vibe it's all about the leaves. Luckily you have the perfect orientation for just such a garden. Here's my pics.

Along the top of your retaining wall plant Alocasia, otherwise known as elephant ears. These are essential in a tropical garden as you need big leaves for a lush look. Alocasia come in many varieties and lots of different colours. Choose the one you like best. They grow in part sun, but can tolerate short full sun hours and nothing looks as tropical as these babies.

Alocasia do send out runners so if you don't want them spreading up there you can pot them up as pictured. Putting these up high will create a canopy-like feel of big jungle leaves.

Next you need smaller leaves and colour, enter an Indonesian tropical plant that is very popular but in your climate will do best with only morning sun. Croton. These are everywhere in QLD and other subtropical gardens. They are slow growing so you never need to trim them and top out under 2 meters high. They have the best colour when planted under taller plants where they receive dappled light for short hours of the day. Too much sun and their leaves bleach. They come in many different varieties, many colours and leaf shapes. They are a beautiful low maintenance plant.

Croton do great as a potted plant as well.

It's not a tropical garden without a water feature. I favour still water features which are simple to put together. The most expensive part will be the glazed bowl which depending on size can run about $200 but it's a one off investment. One of these with either a water lily (not lotus it's too big) or a papyrus, throw in some frogbit which is a floating water plant and you'll have this....

As long as the bowl is less than 30cm deep you won't need council approval or a pool fence. This particular bowl is growing a sedge (tall grassy stuff) arrowhead (the triangular upright leaves, you can eat the bulbs) a small water lily and a canna. Perfect for a tropical vibe.

For something to cascade over the edge I'd avoid the Dichondra silver falls pictured in other answers. They create a dry climate rather than tropical look. Great tropical cascaders include...

Syngonium, you can get this at any garden centre or take a drive around local parks it's often growing wild as a garden escapee. It's very easy to strike. Just pull of a section with a bit of root attached and stick it in your ground.

Hoya, or wax vine. Few things look as tropical. Also very common in garden centres.

Or another option is to grow this native, DIanella silverstreak all along that retaining wall and let is cascade over the edge. It grows full sun to part shade. It's easy to buy in bulk for a mass planting.

Once you've got the big stuff planted up, you'll have patches of sun and shade that you should fill with creepers and other low growing plants that take your fancy. Silver Lady ferns are hardy and look great in a shady spot.

You cannot go past a birds nest either as they famously take even full sun without burning. This ones growing with caladiums (miniature type of elephant ear in very colourful variations and a grass.

While single plant mass plantings look great around hotels and public parks. A backyard is much more interesting if you mix up the plantings with carefully chosen combinations that create height, big leaf drama, colour and texture. Tropical gardens can be very cool. Look at all the different heights, leaf colours and textures here. Creating the tropical look is about creating a canopy effect with some tall plants, big leaves, coloured leaves and creepers.

PS - Forget about bamboo. Someone planted 20 clumps of the stuff in my garden, it's ugly and impossible to remove as the rhizomes are like concrete and need a bobcat to dig out. What people don't tell you about clumping bamboo is that the clumps continue to get larger and larger taking over your garden. It also regularly goes through periods of looking awful.

Black Bamboo, wow! Thank you so much for all that detail. I absolutely live the Elephant Ears plant. Seeing as the top retaining part is only 50cm deep will these work there or get too big? Also love the pop of red and all the different heights. Would you carry those plants down to the bottom section? I do like the idea of stepping stones as suggested by others. I know it's not a big area. but what are your thoughts of incorporating a strip of lawn?

Elephant ears will be fine in that narrow space. They grow kind of like palms so don't need a lot of root space and I think putting them up high will create a more intimate effect than down low but both options would work. If you want them on the lower level just place them in the corners, as at that height they take up a lot of space. Once you shade the place with large leaves you won't get enough sun to grow most lawns. However stepping stones with shade grasses like mondo, zoysia and others will work well. There are even creepers which can be walked on that grow in part shade too. Lots of options.

I would put maybe 3 or 4 elephant ears on the top of the wall. It won't completely shade the entire yard, just create some real dramatic effect. Put some silver streek, or even ixora in between the elephant ears to get the jungle effect. Plant crotons or similarly colourful plants on the lower level. Choose plants of varying heights, some over 2ft tall, foot tall and then creepers.

Tuck ferns into the dark spaces. Plut your stepping stones and plant a creeper or grass inbetween. Chuck a water feature in there and you're done.

If you want tropical effect however, you're going to have to amend your soil. Sandy soil is very free draining. Luckily is is not so hard. Water crystals/vermiculite, compost and coconut coir in large amounts will help create the moist, water holding soil you'll need. Get these from a landscaping supply as it is far cheaper that way, not the nursery.

Thanks so much for all your suggestions. I now have some clarity 😊

Have fun. Gardening is great fun.