The first image that came to mind when I saw your space was this garden:
I love that you already have the circular space planned out. I think that's one of the best features to create in a garden. A few bags of gravel, some taller lush plants that would blend well with what you already have, and a couple of lounge chairs will make this look come to life!

Lots of potential, lovely wooded backyard.

Thank you for all the ideas and comments!

Of course, happy to help! Please keep us up to date with any other questions and progress.

I agree with all of the above. I would use this space as an outside room. Fireplace along one side, or a fire pit or fire table in the middle, seats around, thin layer of pea gravel as a base if you're not staining and sealing the concrete. You could use larger stepping stones to delineate the spaces, accented with some large potted plants. Maybe bring in some sort of structure to add some height to the area to bring your eye up.