I have the 3 3/8" drums brakes as you do.
I find them quite adequate with my controller set on #3 of a possible 10.
I use the least aggressive setting on the sync-control as well.
It is possible that you are a little like some of us, and want a lot more stopping power.
4 years ago I made an adjustment to my ability to stop my 5th. wheel trailer.
I now mostly just let the jake brake do the initial slowing down, then I lightly touch the brake pedal for a complete stop.
My stopping distances are as good as it gets.
My trailer is just over 16,500 lbs.
My truck is just over 17,500 lbs.
You may just want to look at a bigger truck.....
Rick