There are many ways to design your area. Here is a modest way to do it:

Is it a trick of this particular photo, or is the floor swayed in the middle? If it is, place another support post underneath, in the middle, before doing any landscaping.

And do not add any soil. It could harm the trees under which you spread it if it's too thick, and any plants you use should be able to thrive in the existing soil.

And just to say, there is nothing that you are planning that has anything to do with the alpine and rock garden forum.

YOu know I've been wondering about that deck sag. We had pros do it but I agree it needs another post.