This is a bit of a problem because if you start irrigating the bed for summer flowering plants or bulbs then the Acanthus might steal the water and stay green as they do in the north. I see you have something planted in the far corner, though I can’t tell what it is, but anyway my suggestion for accompaniment to acanthus would be a little tree for summer interest - acanthus is perfectly happy in shade. If you don’t want too much height then you could train a shrub into a tree. My first thoughts would be: a small variety of pomegranate, Caesalpinia gilliesii, Yucca elephantipes with its lower trunk exposed or even a single-stem Nerium oleander, but with a single flower because the scented double types droop terribly. None of them need water after the initial settling in year or so.