Vaughn, thanks for your reply.
I tried concentrations and exposures at several different ranges, from 2% to 6% (I use twice as much as solution as you, so my 2% is your 4% in terms of dichromate per square inch).
Exposing longer would give me a smooth tonal range in the highlights, but the image is too dark. When my highlights approach the desired value (zone VIII), there is a sudden drop in density. If I were to draw a curve, you would see a steep drop in the lower densities instead of the typical carbon toe.

Where I would expect to see zone VIII and IX grays I see paper white. So I doubt it's the highlights being underexposed.

It looks like the gelatin gets particularly fragile when it's thinner and falls off, even if I use no agitation in development. I'll post a scan of my test strip when I have access to it.

I also wondered if I was using too little solution and missing spots during my brushing. I prefer 12ml for a 8x10 so I get a more uniform coating, but if you say you use half of the amount of liquid, maybe that is not the problem.