I'm not an expert, never seen bumblefoot as an infection, though I have seen callouses. From what I understand it is an infection. For the bacteria to enter the foot and create an infection there has to be a wound of some kind, a cut or puncture. For you to be getting that much bumblefoot something in their environment has to be cutting or puncturing their feet to create an entry point for the bacteria. I have a lot of trouble envisioning how jumping down onto typical soft bedding could do that, especially the Welsummer which should be a smaller chicken than the Brahma.
I don't know what their environment looks like. Are they scratching in something sharp, like crushed stone that has sharp edges, broken glass, some type of sharp metal? Are they roosting or perching on something that could cause a splinter? I pruned roses yesterday, could they be getting punctures from briars, maybe dead briar limbs. DE works by its razor sharp edges but I'd think it's too small to cause this type of injury but I'm kind of grasping at straws. It may be something like the tip of a screw sticking out where hey can walk or perch on it.
I thought the big risks of big heavy chickens jumping down was more dislocated joints instead of this type of injury. I could see them jumping down as a contributing factor to bumblefoot but not the cause. Something where they land would have to cut or puncture their foot to allow the bacteria in.
I'd expect a wet environment to house more bacteria but I can't see you keeping a constantly wet coop or run. I think I'd concentrate my efforts to try to find what is cutting or puncturing them. Since it could be many different things that may be impossible to determine.