Hey, I think the idea of replacing the Japanese Maple with an Eastern Redbud is a good one. The fact that there are few of these in your area is always a good sign that they will do well in your home. With the tree being a flowering ornamental tree will also give your garden a lively dynamic through the seasons Here is a link that might be useful: front yard landscaping ideas

If I may, I would suggest to build up the rock retaining wall along the driveway a couple levels higher, raising this you will start to level off for your patio. Adding steps on the narrow side will help break up the wall add another design element into your space and will help create a sense of depth to the narrow area. Plant tall vegetation along the street side of the property to provide privacy and create an a more intimate area. Add a stone retaining wall next to the fence but provide space for more vegetation to break up the fence from the house. Continuing the wall on the fence will help you lower the back side elevation to meet your driveway wall height and give you a nice flat space. To really make it feel and look great from the house continue the fence wall out into the patio area in a circular design on the uphill side to create a seat wall and define the area. Good luck!

I’m not sure about plants for your area ... maybe take a picture to a local nursery and see what they suggest for sizes, your soil type, etc. :) (I used a combo of ornamental grass, dwarf shrub, perennials. A dwarf Nandina might work there for the shrub — there are lots of varieties that have nice color all year.)

maybe just mix in a few shovel's worth of sand Sand needs to make up more than half of a planting soil before it starts to affect aeration and drainage. So to produce such effects the existing sand content of a soil has to be augmented with enough additional sand to end up with such a final percentage of sand. Otherwise amending of individual planting holes so that a pocket of different soil texture is created within a larger area can often have adverse effects on recently planted stock due to how movement of water into and out of the amended hole is affected by the involved textural differences. This can result in slowed growth, sometimes even failures. With this phenomenon not being generally noticed due to plants growing away normally once they have rooted beyond the amended hole, into the zone of unmodified soil around the hole that they should have been planted in direct contact with in the first place.