I would certainly include Siberian cypress, Microbiota decussata, in the mix! Moderately fast growing, will take some shade, extremely cold hardy and turns an appealing plummy color in cold weather.

Looks like not many people are replacing their lawns (or part of their lawns) with conifer groundcovers, I wonder why. Is it essentially just a big garden bed? More maintenance in the end?

Grass has it's downfalls for sure, but there are not a lot of plants that can hold up to the usage most homeowners want to use their yards for. I would guess that the majority of people looking for a low-maintenance solution still want to use their yard, so they go with something like clover, or thyme... Not too easy to toss a football on ground cover juniper!

Slopes are where conifer ground cover shines. In my area, I see many houses that cover an entire front slope of their yard with ground cover juniper or Microbiota in shadier situations. It looks great, and functions well to stabilize the slope. Most just go with the big box store stuff, but you can set yours apart by mixing in pendulous conifers and different textures and mounding patterns.

With your zone and lack of direct sun, you probably need to look beyond conifers for anything except green... maybe some thing like Berberis thunbergii 'Bagatelle' for red, and who knows, maybe it's enough the Juniperus horizontalis 'Mother Lode' would take on a nice chartreuse color. A few other conifer ideas:

Pinus sylvestris 'Hillside Creeper'

Abies Koreana 'Green Carpet'

Thuja occidentalis 'Golden Tuffet'

Tsuga canadensis 'Cole's Prostrate'

Picea abies 'Reflexa'