>> I also started a keyhole garden bed last year but never got it finished or filled. This year I will finish and plant that as well.
I think a tall keyhole garden with sandy soil in a dry climate would make it difficult to keep the soil moist unless you had drippers on a timer, running one or two times per day.
My raised beds with porous walls dry out very quickly near the walls, and especially quickly in the corners. I use concrete paving stones stood on end, so my walls let water and air through very quickly: 3/4" or 1" of concrete is very porous. I think it even wicks water away from the soil!
In order to keep soil next to the raised bed walls at all moist, I had to line my corners and some walls with heavy plastic. I use plastic from the bags that bark or manure came in. I line the walls with that plastic but keep the plastic just below the soil surface. Often the plastic sticks out when soil subsides, but I don't mind. Seeing the soil stay moist days longer makes up for any untidiness.
If you're experimenting, you might try lining one corner and one wall of a riased bed or keyhole garden with something impervious. If that corner of the bed is much more productive or less thirsty than the rest of the bed, next year add some lining to the other walls as well.
