Before you acquire any plants, do look at their width as well as their height when fully grown. Since I'm using my phone it's harder for me to get a good look at the space in your picture.
I have 2 very large red oaks and am always amazed at their roots. They seem to extend forever and are fairly shallow. I say this with confidence having stubbed my toes too many times.
I also have oakleaf hydrangeas. They can grow pretty big depending on the variety. Don't know your other plants except for columbines and heuchera. I live in a very different climate and in the heat of the South those plants like a lot of water.
So anyway, you're correct in thinking that you should go slow. That's just my opinion. stone knows much more than I do.
