I'd say mix. Trouble is, that's work. Double-dig - deep as you can - fork, compost, rake, turn, mulch. It took me about three years to make clay soil into a good vegetable garden, but you might have an easier time with ornamentals https:
https://www.hgtv.com/outdoors/...
Another alternative is to go one little patch at a time. Dig a few holes, improve the soil in them, plant shrubs or large perennials, like hostas or Russian sage to make attractive islands. Next year, dig holes between those holes and plant something else.