We have some downhill mountain grades but none that long - the longest is something like a mile and a half. I use L for braking all the time and haven't had any issues with the transmission or engine after five years. Downhill assist uses both brakes and engine compression for slowing but I feel that would not save much wear on the brakes.
Visit Pike's Peak, Colorado. On the downhill road there are checkpoints where they measure the heat of your brakes and remind you to shift to L. (If your brakes are too hot, they pull you over until they cool.)
I tried using 2nd, but it barely provided any engine braking at all.
In "L" I could easily keep the RPMs below 2.5k, and let me use the brakes sparingly. This is actually my first automatic in a long time, and in my previous manuals I would just downshift as needed, and so long as the engine didn't over rev, it was fine. However, I wasn't totally sure regarding an automatic.
By the way, this is the Dolly Sods forestry access road (#75) in WV. It's pretty much 3 miles of constant grade, gaining about 3000' total in elevation. It's unpaved, switchbacked, no guard rails, and a steep drop off the side.
Sure sounds like "L" is doing the job nicely for you. And even if you had DAC it only works at very low speeds, below 10mph or so as I recall, and comes into play in circumstances where the terrain is so steep and slippery there is literally no human way to control the vehicle.
Looks like Forest Route 75 and others are a dirt bike haven for street legal dual-sports like my DRZ. I may have to check it out on the trip from Reading, PA to Orleans, IN next year. Hopefully the speed limit is 50mph! :wink