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If you want big leafed, tropically looking things like in those photos you could try an upper storey of things like tetrapanax, paulownia tormentosa (hard pruned every year not grown into a massive tree), cannas, hedychiums, salvia amistad, tree echiums - all of which will love full sun but most of which will be hard to find in your average garden centre.
I wouldn't plant anything in August, in a middle of a drought and heatwave though - unless you're going to be watering them twice a day. I'd wait at least a month for the temperature to come down a bit and hopefully some rain too.
You may have better luck than me but I've found fatsia japonica doesn't do well in full sun - it won't die but it doesn't look healthy. Mine have grown very well in deepest darkest shade
Doesnt have to be now, I think some more shrubs to add a bit more height/structure, then next year plant some perrenials?
I think the problem is the garden doesn't get much partial shade, which is the vibe I want.