I don't think being at 6 is your issue. Mine likes and is set at 6.5 fwiw. It's also in a school bus, which is significantly heavier then most trucks/trailers on here, and can do 70 mph. From your video, with the little bit of running that I heard, it sounds like it's timed close enough, as I can faintly hear the fuel clatter. If your timing is too retarded, the fuel won't clatter, and it will sound more like a big block gas engine, vs a clattering diesel.
And that 6 degrees is assuming you have the tool set up right. I've got both a j33300 timing meter and a snap on mt257 with a timing light, because I wasn't 100% certain in myself. Using the light was just reassurance that the meter was correct.
Bear in mind, no NA idi will ever be a hotrod in my eyes. Modern trucks are just so far ahead of the old stuff it's not funny. Trailers that you needed a 1 ton diesel for 30 years ago can be done by a half ton ecoboost now, and the ecoboost will do it faster, stop it sooner, and get it done with better fuel mileage.
But, the big thing to a low power complaint is are you seeing black smoke out the exhaust? If not, then you're short on fuel, from filters, clogged pickup, bad fsv, pinched hose, you name it. If you do have smoke, you can move the pump a tooth, and then retry timing it at 8.5. But I wouldn't count on that 2 degrees waking it up considerably.
Pumps aren't cheap. I would be certain the pump is faulty before I threw one at it. If one was faulty, I feel you wouldn't have smoke either.
