You don't necessarily have to limit yourself to one kind of plant that would suit both full sun and dense shade, and I don't think that you will find one. So, can you look at this problem as:
1. I need small trees/large shrubs that will do well in full sun and grow to 5-6 feet, and
2. I need small trees/large shrubs that will do well in dense shade and grow to 5-6 feet?
Then you have lots of options for the first problem. As an advocate of native plants, I would recommend the following:
-Chionanthus virginicus, or White Fringetree which grows in full sun to part shade.

-Clethera alnifolia or Sweet Pepperbush for shade.

-Aesculus parviflora or Bottlebrush Buckeye for shade.

-Viburnum trilobum or American Highbush Cranberry for full sun or part shade.

-Viburnum dentatum or Arrowood Viburnum for full sun to part shade.

-Ceanothus americanus, or
New Jersey tea, for sun to part shade.

-Callicarpa americana, or Beautyberry for sun or part shade.

-Hamamelis spp. (vernalis and virginiana) or Witchhazels for sun to part shade.

I have linked to the Missouri Botanical Garden which is an excellent website for reference. Here in Central VA Zone 7a, we have all of the above growing with no problems. We try to plant for native pollinators, birds, and other wildlife. That's just a start. For further reference, I recommend Doug Tallamy's books "Bringing Nature Home" and "The Living Landscape". You might also get some advice from Virginia Western Community College. Their little arboretum is chock full of inspiration there in the Star City.

Good luck with the neighbors!

These aren't native, but I wanted something about that height for that purpose that isn't deciduous. I planted some golden euonymus, but some winters it's like candy for deer. Two years ago they were all stripped to the stems, but have come back and this last winter were not touched. After that deer disaster I planted four Steed Hollies that are attractive, but too soon to tell if the deer will like them, and also about six dwarf Bufords that they will probably enjoy. I didn't research this in advance, just got what was on sale at the nursery at the time.

Another buffer you might consider are Nandinas. Make sure they aren't the dwarf variety. Deer don't eat them. I have some very 'bushy' ones which hold onto most of their leaves through the winter here in central VA.