some ideas

Why do you need to fill it? Beds don’t have to be full to be attractive.

The accent rocks you have used are too small, though. If you are going to use a rock for an accent, use a ROCK, not a basketball sized stone.

You need some variety of shape, too. A grouping of short ornamental grass, no more than 3 feet tall, would be good. Or even a clump of spike shaped flowers would work too. Like a salvia.

Hard to tell in the photo, but it does not appear that shady. Perhaps it is a soil or water problem.

It is bright shade. Our house faces northeast and our neighbor's house gives us shade (we are in a cul de sac). Perhaps it is a soil problem. Our heucheras and boxwoods have been fine though.

Do you know what kind of hydrangea or salvia you tried? There are hundreds of types of salvia, including at least one that likes shade, though it isn't appropriate here. There are also 4 main types of Hydrangeas and each has its own requirements. Hydrangea macrophylla cultivars, the most commonly sold type, really doesn't do well in zone 5, even though it gets sold here. But H. arborescens like 'Annabelle' and H. paniculata like 'Bobo' or 'Little Lime' are fully hardy and do well in zone 5.

It sounds like you've tried a bunch of plants. What went wrong with the ones that didn't do well? Did they die or get smaller or just not get larger quickly? Have you gotten a soil test done for this area by the local cooperative extension service? That will give you info on soil texture, pH (which is really important to what will thrive) and any nutrients that are lacking. Often with new builds the soil has been both disturbed and compacted and that may need some remediation. Having good soil with lots of organic matter such as good quality compost turned into it will help virtually whatever you end up planting.

You say it is shady, but it doesn't look like really deep shade. Does it get any sun? Are there any overhanging trees or is it only from buildings? How close are the buildings that create shade? What do you do to water the area?

Can you add photos of the whole house front and include the edge of the neighbor's as well so that we have a better idea of where everything fits in and how it influences each other? This area needs to relate in some way to the bed on the other side of the walk way, so having a photo of what is there will help.

I agree with Littlebug that your rocks are smaller than they should be for good ornamental value. Doug also drew larger rocks in his sketch, so consider either removing the rocks entirely or replacing them with ones that have some real substance.

My memory is that in previous discussions that the general feeling was that this is an area that would be best planted mostly in lower plants since the porch and stonework of your home are so nice. I wouldn't plant anything to hide those beautiful stone pillar bases, or anything that grows higher than the base of the window. The exception would be a narrow shrub or vine such as clematis on a trellis near the corner of the garage. You may want to use a relatively limited number of types of plants since this is a small area. The Heuchera/coral bells are doing well, so have you considered adding some more of them? Do you have deer, rabbits or voles? That will also influence what may thrive here.

Some plants to look at include

one of the small Hydrangea arborescens, especially Invincibelle Mini Mauvette 'NCHA7' which shouldn't get taller than 3'

Epimediums which come both as spreading plants and as groundcovers

Hosta (if you don't have deer)

Hakone grass AKA Japanese forest grass

Mukdenia

Pulmonaria

Ligularia

ferns, especially Japanese painted ferns

Siberian iris

meadow rue, which have relatively low foliage though having tall flowers for a while would be fine

perennial geraniums

Veronica 'Georgia Blue' as an evergreen groundcover

Perhaps one of the smallest rhododendrons such as PJM Checkmate which stays around 3' tall.

Having one other type of shrub to contrast with the boxwoods, some additional Heuchera, one or two other perennials, and an evergreen groundcover to help tie things together would be enough variety. Be sure to leave space around the boxwoods so their growth isn't slowed down by competition. You may start seeing a slightly faster growing rate for them this year since they should be established after 3-4 years.

Once you have an electrical outlet right there, I'd add a nice mid sized fountain. Mix in some annuals with what you already have and your done

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2nd design has ferns, geraniums, dwarf yews, allthea