yep, highway miles is different that city miles.
I used to turn the brakes almost off in the city back when I had a cheapo controller. I just did not need the braking power to stop fast when speeds were low. On the highway I just never needed to do a lot of braking, so my brakes could go miles and miles.
Now that I have a high-end controller I leave it alone, it brakes softly when the truck does, and brakes harder when the truck does, less wear than a timed controller that brakes soft a second, then increases braking forces over a few seconds. Best $200 spent.
I would not replace the drums until they need replacing. $400 (4x100) might be pocket change for some, but most would rather not toss that money away needlessly, and I would never go disc on a regular trailer. Drums are fine for most sized trailers. DIsc are wonderful, but I cannot justify the expense when the drums are more than enough. Most tow vehicles do not need trailer brakes to begin with, which of course is of opinion. I still cringe to think dear old dad towing the family trailer with the station wagon equipped with 4 drum brakes, on the car, not the trailer, none on the trailer.