My favorite edging - Steel. Its strong, bends, changes color, you can seal, paint it, takes up minimal space, can be recessed or use to create steps. My 3 recommendations, 1- dont use a thin (gage) steel because it will bend when people step on it or to thick that you cant work with it . 2nd if your budget is tight check a local metal shop, see if they have cut scrap pieces you can buy. 3rd hammer the furtherest corners of your edging to be safe.
use vertical railway sleepers to create a curved wall raised beds - curved walls may work with overall design better divide between Mediterranean garden and English garden rather than original oblong med wall bed idea