Harness won't help you train anything. Sorry, I've dealt with people that think harness is the way to go, and it takes them twice as long to teach the dog anything.
Pressure on the neck? If a flat, prong, martingale, do any damage to your dogs neck...its YOUR fault, not the collars. If your reason for avoiding a collar is so that you don't hurt your dog with it, that's fine, but IMO a collar won't hurt a dog. I've got an 85lbs male, if you think you can control that dog on a harness...good luck. At least with a collar I can redirect his head, give an actual correction. With a harness, either he drags me around, or I have to plant myself in one spot to make sure he doesn't get to whatever he wants (bite work).