First off, don't let "Dog Guy" fool you into thinking I'm a trainer. I spent a lot of years as a SAR dog handler and I was very good at that, but I've never had the expertise to train someone else's dog. I've just picked up some useful tools along the way.
On prong or slip collars: there's a misconception that these are hard corrective tools or only suitable for hard dogs with problems. We rarely needed to use them with our GSDs, but have needed them with all three of our Labradors. The GSDs figured out on-lead corrections and behaviors pretty quickly. Our three Labradors have been more knuckleheads. They'll happily lean hard into a flat collar and haul away. Labs are also prone to throat irritation from exactly this behavior so the slip or prong allows a correction that's less likely to harm the dog. The end result is a walk where both the handler and the dog are happy and relaxed, and the dog doesn't end up being treated for an inflamed trachea.
Regarding timing of reward or correction: Check out "Don't Shoot the Dog" by Karen Pryor. There is much more to training than timing and reward but I found a lot of good tidbits in her book.