towing near max weight. advice?

That same thing applies to GVWR's. There are multiple GVWR's for the same brakes, axles, etc.. 9,000 up to 12,200. So which ones are the brakes designed for??

Good point on the many GVWR especially Fords F350 SRW with over a dozen different GVWRs but the same brake specs.
Brakes are designed and tested by the truck manufacturer before they do the FMVSS testing or so says Dodge truck engineers at the '03 Chicago truck show when Dodge released their newly redesigned '03 2500/3500 trucks. FMVSS testing is just a minimum test requirement that a vehicle has too pass and has nothing to do with a vehicle s actual max braking performance as the truck maker can any assign GVWR to the vehicle as long as it doesn't exceed the sum of the vehicles GAWRs.
And as in my repy above NHTSA says the brakes to tied the axles systems. (see my reply above).

I cannot speak for every trailer axle out there, but I know that Al-Ko uses the same 12”x2” brake on the 5,200 thru 7,000lb axle. I have seen similar stuff in Dexter catalogs. So that’s not always true.

AL/KO says this about brakes/axles

"Weigh the trailer after it is fully loaded and ready for use.
Axles, brakes, wheels, tires, frames and suspension components are
designed to carry a specific maximum weight. Locate the VIN
(Vehicle Identification Number) plate on the trailer. It will show the
GVWR (Gross Vehicle Weight Rating). The GVWR is the total
amount your trailer (including tongue weight) can weigh when it is
completely loaded including holding tanks, propane etc. The GAWR
(Gross Axle Weight Rating) is the maximum load that the axles will
carry and the maximum load the brakes will stop."

Now if AL-KO has used 7k brakes on 5.2k axle then thats great but you can be sure the brakes are not less than the GAWR unless someone at the factory installed the wrong brakes on the axle.