Bunchberry is tricky to grow, even here. It prefers mostly shade and a rich, evenly moist soil and seldom develops into a very dense groundcover. I'm not at all sure it would thrive in your situation. And neither it nor the maianthemum will be evergreen.

I'd consider kinnikinnick (Arctostaphylos uva-ursi - native), salal (Gaultheria shallon - native), Oxalis oregana (evergreen form) or a couple of non-natives like Vaccinium vitis-idaea ssp. minus (lingonberry), wintergreen (Gaultheria procumbens) or vinca. The pachy would work well also.

What about low growing mahonia? Alpine strawberries? Western swordferns? Here are a few more ideas: https://www.wildflower.org/expert/show.php?id=8540

Thank you both for your very helpful comments.

I think I am going to try wintergreen (got a few cute little ones today, complete with berries...will see how they do before buying too many more) and kinnikinnick (since I looked it up I realized I've already seen it growing in some harsh places in my neighbourhood so I'm sure it will do fine). I am definitely going to include some western sword ferns but those grow gigantic - I would like something low and spreading below.

@gardengal48, can you comment on how fast wintergreen and kinnikinnick will spread? Can I be cheap and buy fewer plants? :D

Wintergreen does not spread very fast at all and kinnikinnick only moderately faster. To get full coverage and decent weed suppression you would need to space them relatively closely. The typical recommended spacing is 18-24" but you could go closer for faster coverage. I'm not sure you'd be happy with the results of a wider spacing although I understand that if a big slope, economy alway factors into the equation :-)

Because of the kinnikinnick's growth habit, you will still get some weed development (although the mulch will help), so be prepared to do some weeding until it is established. And keep on top of the weeds going forward........it is almost inevitable that you will get some alder seedlings or blackberries popping up and whatever else might be common in your area and if they get a toehold into the slope and groundcover, it will be far more difficult to remove them than if without any groundcover. Unfortunately, NO groundcover is 100% effective at weed suppression.

Thanks for the detailed reply! Very informative and helpful