You can fill in some ferns between the hostas.

Brunnera can take very dry shade conditions and has much the look of hostas with mounds of overlapping large leaves. It usually appears on lists of plants suitable for planting under maples. I don’t have a maple here but do use ‘Jack Frost’ ( actually mostly seedlings from it now...) in dry shade under the edge of pines. Christmas ferns also grow well in the dense heavy shade and very dry conditions under the pines and also appear on list of things to try under maples. ‘Stinking hellebore’ is another plant that appears on lists for dry shade under trees.

I'm in the PNW with a large maple with surface roots in one corner of my yard. I've tried many plants that supposedly grow in dry shade, and found most very difficult to establish. The only plant I've successfully grown directly under the tree canopy is epimediuim which seems to thrive in the harshest conditions. The farther out from the tree trunk I go the more I've been able to grow hostas, brunnera, ferns, pachysandra and hellebores. You might also try some other tough ground covers that are sometimes too aggressive in less hostile conditions (bishop's weed, blue star creeper, etc.). My approach is to see what survives and then plant more of that. Good luck.

Best plant for dry shade is Geranium macrorrhizum.

All good suggestions above. For vertical interest our Northern Sea Oats grows well in shade with maple root competition.

Here are some half-baked suggestions for organization. I can't really see anything tall, per se, next to the deck. But the tallest material of what's there I'd place at outside corner(s), The ongoing color, I'd place next to steps.

I can't see the lawn extending into the nether reaches and shady areas (under tree) as being practical long term, and would divide the area into lawn vs. groundcover. Would make no bed shallower than 4.' (I know ... some in the picture looks 3' because too lazy to redraw.) Would run a path (mulch, stepping stones, walk, etc. ... whatever suits) through groundcover to connect the lawns. The groundcover would not necessarily be the same everywhere, but what suits a particular area, which could be subdivided as you see fit.

Thanks all for your advice! Ideally, would love to plop a japanese maple in the deck corner where the pole for the hanging baskets are. That's probably the more "rooty" area of the whole border, although I could probably fanangle it.

I will probably do a mix of heucheras, fern, hostas, epimedium, and geranium. Never planted epimedium before... I just looked it up, good suggestion! Dark beauty is pretty!

Check out hellebores. I have some that look like this and while they prefer moist shade they’ve done fine for me as long as I monitor watering, not needing to hover, just keeping an eye on things regularly. They add nice contrast to ferns and hostas.

Lanium is a ground cover sized perennial plant that blooms all summer, a rarity in perennials. Cones in several colors.

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If that's a Norway maple you're not going to be able to grow squat under it including ground cover. Even if you add raised beds, the tree will just fill them with its roots. The solution, other than chopping down the tree, is to grow things in big pots under the tree. Maybe make a little gravel island with three pots of various sizes, or something like that. Same for by the deck, you could build some raised beds there. Japanese maple, particularly if it is a small one, can be grown in pots and are lovely. You have a lot of options but I love the glam of white hosta, red astilbe and blue brunnera.

Personally, I would pick out a more spacious, sunnier place for a Jap maple than near the deck corner. They can get quite large and will provide better fall color if not in too much shade.

Best plant for dry shade is Geranium macrorrhizum.

Yup.