Not really what you asked, but I am really not a fan of harnesses for normal dog walking. Harnesses are for pulling, collars are for walking.
I especially wouldn't try to use him for dog assist on uphills unless you are OK with him pulling on his leash normally. I've managed to train dogs (Sled dogs and house dogs both) the difference between harness (pull now) and collar(don't pull) but it takes some work and a dog that wants to learn. I watched a ton of people try and skijour with their house dogs and destroy good leash manners.
It has been my observation of the dog folks in FL at least that harnesses and gentle leaders both are crutches for folks that are unwilling to train their dogs properly. Not to mention that if the dog does see something that sends all their training out of their little doggy brain a harness gives them a much better platform to jerk you around with.
YMMV
My house dogs, when they were young enough to hike, carried their own snacks and water in one of these: https://www.amazon.com/Mountainsmith-K-9-Pack-Caribe-Medium/dp/B07MQ3J4GV/ Although mine is not quite that cool, as it was several years ago model.
There's also room in there for spare mags.
No, you make good points. I absolutely don't want a harness for walking. I've already got him loose leash walking well (except we're working on distractions now in dog school) with a regular collar and leash.
I think he's smart enough that I could teach him the difference between "pull" and "no pull", based on him recognizing that collar and harness are different, but you make a good point, and I absolutely don't want to lose his loose leash manners, especially since we're currently refining them. Given that at home he has 40 acres to roam on, he's been really good about knowing how far he can go leash free, and what he has to do once the collar goes around his neck. I'd hope I could transition that to the harness.
Though the pulling me up slopes was more of a joke, but I expect that on rough steep trails, some of that is going to just happen given the terrain, which is why Iwas looking at a harness, as I would have a fear that slipping or otherwise tricky terrain could lead to throat damage with a collar.