I had two 5200s on my 5er about same size and weight. No problems with braking for the trailer at all. In fact, for a time I didn't even know the trailer brakes were bad---the truck was doing it all and I couldn't tell. ( I admit I failed to do the brake test using the slider on the controller with my foot off the truck brake)
So IMO if you are having trouble with 5200s and plan to spend a bunch going to new axles and all that, don't! I would be looking at the truck.
Is the truck heavy enough and how are its brakes? My 2003 Chev 2500HD was able to keep things seemingly ok going down steep hills and there were no trailer brakes at all!
Also note that the trailer is supposed to get braked first then the truck so the trailer does not keep coming after the truck stops--didn't happen with the 5er anyway---and the trailer is not supposed to "lock up" and skid when you do apply the brakes.
I suspect you have a non-problem with the 5200s, but maybe do with the truck and its brakes.