I’d definitely check to see what’s local to you, but I have a couple of ideas mostly relevant to my area in the Midwest, which is zone 6a/5b.

A lot of spring ephemerals like shade - trout lily, mayapple, trillium, and Jack in the pulpit come to mind. Wild geranium and ferns prefer shade.

A lot of shrubs are shade tolerant, including sweetfern, coralberry, spirea, gooseberry, bearberry, viburnum, mulberry, and pawpaw. In my area most of the understory is spicebush, sassafras, and dogwood.

Hope that gives you some inspiration!

Depending where you live, you may have a native plant society or something similar that could be of great help.

I’m a sucker for ferns so I’d cover that yard in ferns with walkways throughout.