I figured you would have covered all of the basics since you said you had done training....that's tough! Chase didn't like the treats outside either....he actually spit them out.
I wouldn't push the Halti if he is really not having any of it....I only use it as back-up now. It seems to remind Chase to be good and keeps him calmer.....but if he was really pulling severely I wouldn't use it anyway because it can cause neck injuries by jerking the head to the side.
Where I found the most improvement with him was with the use of a martingale collar (NOT a choke), using a quick leash correction anytime he pulled ahead, praising him whenever he stayed at my side even for a split second, and continuing with abrupt changes in direction. I also did umbilical (leash around my waist) with the martingale, and attached Chase to me while making dinner, vacuuming etc. - I aimed for about an hour a day, and it helped teach him that he needs to go where I go, and that the leash doesn't equal going full steam ahead. The only problem with the martingale is you have to be ready for him to pull and correct before it happens.....it doesn't minimize pulling like the Halti does so if they get enough momentum and pull away from you, there is not enough slack on the leash to give a correction anymore AND you can end up with some nasty rope burn! Oh, and I also practiced basic things like he has to be in a sit-stay before going outside, I go out the door first, and he needs to be invited out.....things to drive the point that you are in charge.
Just remember that whatever you try, progress can be very slow. Chase is 13months old, has taken nearly five months to show real progress, and we're not all the way there yet. He was so bad on leash and got so strong at 7 months I realized I was shying away from walking him, but once I decided just to keep practicing over and over again he started to improve. It can take a lot of time and a lot of consistency.
Chase (Friendly Resident Wigglebum) - Border Collie/Lab/Shepherd X
Kailey (Misunderstood Gentle Beauty) - GSD & foster failure #1
Rupert (Gold-Medal Winner of the 3 a.m. Kitty Destruction Olympics
Heidi - RIP my sweet baby girl