Before you do anything else there is bindweed there which needs to be eradicated completely or it will be a perpetual pain. Every scrap of root needs to be found and removed.

What is the dark green plant already growing there? It seems happy.

I would get rid of the "terracing" stones, as it is a busy look. A simple slope that corresponds with the steps grade would look better. And a single, uniform groundcover would look better and be easier to maintain, than would collection of different plants. If dwarf Mondo grass (Ophiopogon japonica) would work there, that's what I'd plant.

Trim the trees so that they do not stick out in the path. Having them be overhead is fine.

ideas:

Yes take care of the bindweed! I could see hakonechloe cascading down the side, covering the flagstones so you don’t have to remove them. Or a prostrate arching cotoneaster..

There are a few plants that could do well in your space. Japanese Painted Ferns are unique and don’t get very large. There are varieties of Heuchera including "Plum Pudding" and "Palace Purple" that might work well. Other options include Hostas, Brunnera, Dwarf Hydrangeas, and Fragrant Sarcococca. Visit Monrovia.com, copy/paste the names of each plant in the search bar, and you'll see pictures of each of them. Would love to see which ones you end up choosing. Good luck!