The slope is a factor for safety, but this view the house won’t look what I consider balanced since it has the second floor porch areea on one side, but that doesn’t mean it won’t look lovely. Once you have plantings, whether in planter boxes or not, things will look different. Right now because it is so bare, the elevation changes are more stark.

For the material of your planters, I would choose one of the materials used in your house, either the stucco or the wood slats. If wood, use a liner to keep soil from resting against the wood so that it lasts longer. It will still need replacement more often than a masonry planter.

To have the forum be most useful, you need to corroborate whatever issues you bring up, with pictures. We need to SEE whatever problems you mention, such as "...where it's about 6' below the slab ..." in order to be able to evaluate what you're asking about. (The picture above does not show this.) When you have an idea, such as "retaining wall/planter box," you need to sketch out a VERY SIMPLE plan that shows us what you're thinking about, in order for us to know if you're on the right trail, or the wrong path. (The plan needs to include only the relevant areas ... not the whole back yard.) The plan and pictures should corroborate each other.