Okay, I'm going to give my two penn'orth about equipment for difficult pups. It may or may not be relevant.
A lot of it is down to the size and strength of your dog as an adult. I suspect that @DowntonNabby has what will be the biggest dog here after Brie and possibly Libi...
I used a harness a few times when Brie was little; mainly for my own peace of mind, but I used it in conjunction with an Ezy dog double up collar and a double ended training lead. That was great because the training lead had a handle at both ends, thus one was next to the collar and one next to the harness. It did give me stability and reassurance in traffic or when encountering prams, small children etc. where it was imperative that puppy did not jump up. However, the harness seemed to be impeding her shoulder movement so I put it aside by the time she was 5 or 6 months.
Interestingly, when we got our previous two pups, both male, their breeders were 100% against the use of a harness - reason being that with a strong dog, it just aids their pulling power. Hence, recommendations of conventional neck collars or headcollars. Obviously a headcollar is not recommended for a puppy under 9 or 10 months so I have not tried it on Brie. We had a Dogmatic headcollar for Algy (who was huge). We found it gave good control for pavement walks on dark evenings, particularly if it was wet or snowy and slippy underfoot. Most of the time he was walked on conventional webbing collar and lead.
I won't use a slip lead because I don't have confidence in them. We've used them in the showring and one of our girls was adept at getting out of them. They also tighten too much with a hard puller and can damage the trachea. I'll be told that I am wrong by 25 people now but none of them will have a LGD, so 😛My husband is not averse to putting a half-check/martingale collar on Brie occasionally but I don't like it. In any case, they wreck the coat.
Miss Fat Arse is 7 months old and weighs 38kg - she can also be a demon puller. I'm on an Ezydog Double-up collar and a 5 foot webbing lead with a heavy duty trigger hook, if that's any help to anyone with a big strong dog. I think I have more faith in that set up than anything else, the lead gives me enough length for field walking but is not too long for pavement control. I always use both hands on the lead because I'm diddy and pup is massive.
Ironically, this afternoon she walked brilliantly for me and didn't pull at all (yay), but there were no little kids and no big dogs and that's generally when she gives me issues. She just absolutely adores small children and wants to make friends with most dogs.
Hope this is of interest in some way or other 😊