You have a great start on making your area a Japanese Garden. You should find and buy some Japanese Forest Grass. Several small clumps near the rocks would look nice. This grass disappears in the winter and reappears gorgeous in spring. Dwarf maples are those less than 8' - which includes weeping lace leaf varieties. Check out dwarf full sun JMs for zone 5a and remember they will grow larger and wider, especially weeping lace leaf maples

I like the look of carefully pruned green shrubbery in Japanese Gardens- trimmed to look like floating clouds. There are better shrubs for this than others. Dense growing and better yet evergreen shrubs are ideal. In your zone you still have the possibilities of both but they can be a little limited (and I feel this pain, I live a zone colder!). Try adding some boxwoods like Sprinter for an easy shrub that seems to love pruning. A few would work well in both areas you have photos shown for.

You could also go the even simpler route and use a beautiful, lighter colored rock mulch and have it raked smooth around your well placed stones, with some shrubs or a larger prostrate shrub here and there.

Here is a link that might be useful: Sprinter Boxwood