I have 2 silkies along with 6 other mixed bantams...their coop is 18" off the ground. At 4 weeks old they were introduced to an open pop hole and the big wide world out there. It took them 4 days to get up the nerve to venture outside and now they spend most all of every day outside their coop. When I wanted to train them to come back up the ramp in the late afternoon I brought a tray of dinner treats (finely chopped veggies, dry oats, etc.) inside the coop and called them. It may take one or two a bit longer to "get it" but by the third evening all of mine came up and in on their own.
I have seen insanely steep ramps leading up to coops almost 6 feet off the ground (in England and elsewhere) and chickens going up and down like it was the most natural thing.
As long as there are plenty of rungs for them to grip to, they will flap their wings a bit and scoot on up and into home.
I have pretty much the same set up as you with the steep ramp. What we did was cover the ramp with indoor/outdoor carpet (no rungs) so my cochin bantams wouldn't slip going up or coming down. I guided them up the first two nights and after that they would walk up and down that ramp all the time.