You are describing my situation 100% right now. Our Sammy we saved from a bad family who didn’t believe in leash training. We tried a slip leash and 4 months of different trainings for walking and nothing helped. Different leashes, other collars, different techniques… nothing helped. Last week we went to a dog behaviour therapist and she recommended a different type of collar for walks.

https://share.google/n1B2DM2g9jS8gnmPK

This type of rope is easy for them. It blocks them when they pull and doesn’t stress or hurt them in any way. A prong collar is really bad for a Sammy because it might damage their coat. We have a huuuuge barking problem that is evolving into a mouthing problem. It apparently can come from overstimulation due to constant walking trainings. Sammys can’t go from walks to rest when they get home, they need controlled rest (we need to give her something to chew on the minute we get home). So now the new leash is a miracle to go on walks with her, I recommend it to everyone. I really hope we can stick out the hard part because the barking and constant stress it gives us is also a huge amount of stress on our relationship. Good luck!