If you say another raised bed is not an option, but don't forget about vertical gardening in case you needed more space. Many herbs and small veggie plants can grow just fine in a vertical garden type of setup.
On a smaller scale, many people do what you are describing when they dig that 12-18" hole to transplant some new plant in. They make that hole with such nice growing material. Occasionally, the plant struggles because if the root grows beyond that 18" radius, it meets a new kind of soil that might make the roots struggle. I have forgotten the term for it but that is a cause of some plants struggling during their first one or two years in that spot.
However, your hole will be much bigger, and for smaller plants that might even die off each year, so it may not face the same problems. I just bring it up as something to be aware of when the plants might be facing a kind of border between two drastically different kinds of soil types.
