I'm not scared of the steepness per se. Not the width either, just because it's really short. The problem is transitions. That would be a
hard transition at the bottom. You want to have a transition that's at least bigger radius than your biggest tire, but that would still be "sharp" at the axle... preferably more like 8 ft radius which is skate-ramp standard going into it. Or maybe like a 3:1 slope with a bit of steel right at the bottom for the transition. At the top the minimum goal is not to bottom out... if you are pedals level that's pretty easy but if you are pedal down or pedaling it's not much. I don't know what the high-center clearance is on a bakfiets, I would guess not much. On another piece of paper, to scale with your steps, draw two 27" wheels that are 40" center to center and a 15" circle between them that's 3" below centerline of the wheels for the pedal. Then "ride" that along your steps, see how it looks.
I've looked at building skate / bike ramps (for, uh, my kids, *cough*) and the "standard" construction seems crazy. They build with 3/4 plywood sides and the joists held to the plywood with screws. There's very little holding each joist up, it depends a lot on the strength imparted by the deck, which is two layered sheets of 3/8 plywood. The ramp is then finished with MDF if you're cheap or with a polymer material or some special outdoor paint if you're not. It's sort of half monocoque. If you compare it to how a house or deck is built to code with all the load from every point stacking into the foundation with no reliance on any other piece in the chain to take any weight, it seems wimpy. But at the same time it needs some kind of sideways strength, which a floor really doesn't, so it does make some sense.
Some sketching indicates 1:3 might be a good maximum pitch for a regular bike. That more or less matches a kicker ramp. Don't know about that bakfiets though.
Look here:
https://diyskate.com/
and here
https://www.ocramps.com/
Also... strollers and wheelchairs and walkers and even recumbent trikes are designed to fit through a 32 inch minimum doorway. That in turn requires a 36 inch wide ramp surface. And a 1:12 pitch. This probably supersedes your CROW manual...if you have room.