Not sure I have a good picture of the geometry, but it sounds like what you're saying is that you really only have a one foot diameter planter with any depth, and it has a wide very shallow shoulder. Water won't perch on a shelf, so don't fixate on that. Strawberry roots are shallow, but they go down a lot more than four inches. So I'd just write off that shelf entirely and plan for a one-foot diameter planter.

As long as the bottom of the whole thing is open to the ground beneath so that water can seep out, it will come off of any intermediate shelf by wicking action.

By definition it is a planter, and should only be filled with potting mix. Garden soil would become too heavy and the roots would starve for air. Strawberries would probably work. If not, herbs, arugula, lettuce, or annuals would be good if someone is prepared to water it often.