I'd thought about adjustability, or lack of, but I suggested it anyway.
I'm not sure how to describe the situation mathematically, but it would involve a mass (the variable load) and a lever arm (the wheelbase). If the frequency is more affected by the wheelbase, might adjustability not matter? The sprung wheel complicates it.
Possibly a rubber torsion mount at the bumper, both springing and damping and progressively limiting the range of motion at the hitch would force more effort from the wheel's spring. IOW, loose at the hitch and tight at the wheel vs tight at the hitch and loose at the wheel.
