50cm width would be plenty, if it weren't next to a wall. I'd say make it 0.80 m, which then leaves 1.00m for the steps which should be ok for two people to pass each other.

Regarding the steepness - as there are only 3 steps to clear, you could get away with a shorter and steeper ramp than in other instances, but the limiting factor in your case is the ground clearance of the bakfiets (it's not so much about the slope itself, as the upper edge). 1:5 or 20% slope should work fine, but that would mean the ramp has to be 5x0.45=2.25m long. I don't know if there's enough space available. If the space is very limited, you could probably get away with 1:3, that makes it 1.35m long, but check to see if the bakfiets can clear that.

Mind you, anything steeper than 1:10 is unsuitable for wheelchairs, at least by our regulations.