The problem with stuff off the back is not the weight, but the torque put on the thin-walled bumper tube.
I made a reinforcement to take the torque off the bumper. This is a couple pieces of thinwall (16 gauge) 2x2 square steel and some gussets, welded to the plate of a bumper receiver. (What looks like a square tube coming out the left is just a piece of scrap it's resting on for painting). I carry a Pelican cooler on a platform, which weighs more than a couple of bikes. It doesn't bounce around, the bumper doesn't flex, no problem.

The receiver part bolts on the top of the bumper, which gives the bikes/platform more clearance.

Any torque gets transferred to a frame crossmember.
