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You've probably realised this, but you need something big and high to give partial shade.
But you could do it with a pergola, a sail or some sort of permanent object.
Verbina mixed with salvia would give you different heights. Verbina in particular can grow through stuff. My mum usually has a big leafed mint and Verbina combo that looks really nice. Also there's one Verbina that's a fucking massive version and looks basically the same as the smaller version.... I want to say Argentinian? Anyway you could use the two of them to give variation of height.
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.