Your most pressing problem is the Silver Maple. These have extremely invasive roots-I cannot stress this enough-it is a challenge to grow weeds anywhere near their vicinity and these trees root out a very long ways. It is my opinion that the tree is way too close to your project area and nothing will grow successfully for very long unless it is in a pot. I had one of these trees-was quite large, and finally resorted to cutting it down because I could not keep it from vigorously rooting into my flower bed which was 30 ft away. So many roots that my plants were being choked out, shrubs even were not making it.
You might get buy with putting down some type of underlayment -then soil -then plant, but that will only last a few years I think before the trees roots find their way up into your rich, watered soil and take over.
If removing the tree is not an option (which is very pricey so probably not?) then I would create a plan to use only hardscape items. All plants would be in planters, I would get 2 or 3 very large nice planters. Smaller pots would stay all the same color (which looks like you have done that) and then some neat rocks. I think a bubble water feature would look nice-where the water looks like it is coming out of a rock. This does not splash around so evaporation is minimal and the small pump for those does not cost much to operate. I think that your area lacks a main interest visually, it needs a large focal point that draws the eye.
Getting grass to grow under your tree is going to always be a challenge. If it were me, I would mulch the area around the tree or gravel that area.
