Wow this has gone insanely complex, I think more then actually needed. I agree bad crimps and open to the element are the bigger issues. I but in all the years and all the trailers I have owned, wire gauge has never been a determining factor for if my brakes where operating properly or not.
I think you can save yourself a bundle of headache and hassle by just properly redoing the brake wires. I don't see any need to get into 10-12GA wiring for this. Just clean crimps or if you must home runs will be a lot less hassle, cost and give you the same results. If you prefer home runs and want to clean up the wires on the trailer, consider a
Weather sealed 7 pin junction box At least that way you can home run your lines if that is really what you desire. I use these boxes for more then trailer, I have one mounted on the frame rail of my truck with wires from the upfitter switches and a nice 10GA ground so I can easilly run wires for work lights, my beacon, etc. to be powered by one of the upfitters.
But when I redo my car trailer one of these will be installed since I pretty much need to rerun the whole trailer, after 10 years the current wires are just getting dry and cracking.