With a narrow bed it can be useful to create an illusion of varying height coming forward from the back, though in reality the narrowness means the plants will be side by side. Nothing too tall as you say.
I would use some smaller evergreens for structure and shape, then add colour and texture to the front by alternating the planting. A few ideas but you won’t have room for all…
Taller plants..
Dwarf conifers… some will tolerate shade, especially if they get summer sun. Choose bright green or yellow tinged leaf.
Small leaf hebes - such as hebe emerald gem for a neat shape that needs little pruning (or box balls buxus).
Some hebes have red edged leaves or variegated for winter interest. A number will tolerate some shade.
Gaultheria procumbens for winter berries.
Bergenia for interesting colourful leaves.
ornamental grass with interesting plumage.
you could also use stonecrop sedums leaving on the flower stalks after autumn flowering has finished for interesting architectural shapes which look lovely in the frost.
In between plant a variety of heucheras which come in many leaf colours from lime green to deep red,
Primulas for long lasting colourful flowers. I find it’s best to buy these new in autumn and replant every year (Aldi/Lidl).
Intersperse with Cyclamen Coum and dwarf early flowering bulbs - the advantage of dwarf varieties is you’re not left with straggly untidy long leaves after flowering, which are more noticeable in a smaller bed and take up valuable space.
I would also plant a trailing bright variegated ivy to trail down from the bed and perhaps Creeping Jenny. Both will need to be kept in check.