How much can a bumper support

You didn’t say how long your ramps would be but your drawing shows that you will have a high-centering problem. Also, that whole wood assembly will need to be anchored to the bed of the truck otherwise as you back in the rear wheels will be pulling hard to get you up the ramp and the plywood will shoot out. To take some weight off the bumper you could add another sheet of plywood to transfer some more of the weight to the bed. That’s going to be one scary ride tho - a fast ride up the ramp without falling off followed by an instantaneous stop at the top. Not to mention trying to drive it up when it’s raining and everything is slick. Must be a better way. Maybe winch it up?