Leash biting nightmare!

If this is answered in the guide please delete. I did read through and didn’t see anything that really helped with this.

I have a 13 month old Rottweiler who is overall a really good dog. He is very friendly with people and dogs (sometimes too friendly), steals things occasionally, but we have made real progress on those with training.

The main issue that I cannot get past is leash biting.

He has done it since he was a puppy and it has only gotten worse as he has gotten older. When it happens he bites and tugs the leash hard, tangles himself up, and has hurt himself doing it. It is honestly getting dangerous, embarrassing and frustrating at this point.

Things we have tried consistently:

• Playing or exercising before walks • Bringing toys on walks (tug, find it, etc.) • Two leash method and dropping one • Standing still and marking when he stops • Giving him a job (commands fall apart once it starts) • Ending walks immediately when it happens • Chain leash • Bitter spray and calming spray • Backyard leash work • Letting go of the leash (he picks it up and chews it like a toy) 

Triggers seem to be U turns or engagement based leash training, seeing people or dogs or animals, but mainly literally nothing. A lot of the time we will be walking calmly and he just looks up and starts biting the leash. I’d even argue seeing other dogs or people sometimes helps him stop more than be the start of it.

It feels like overstimulation or frustration and the leash becomes his outlet.

For context my training style is balanced but heavily focused on positive reinforcement. I am not going to hit or physically punish him and I honestly do not think that would fix this anyway. He has hurt himself doing it before and still repeats it.

To give an idea of his overall behavior, a trainer came out for a different issue and said once we improve his neutrality around people and dogs he would likely pass a CGC. His impulse control is good. Down stay until released and is actually better off leash than on leash which makes this even more confusing (heeling off leash, recall, etc)

I am not looking for “go back to basics.” We do structured training daily 3-4x a day. I am looking for things I may be missing or something that worked for others with a dog like this.

Has anyone dealt with persistent leash biting at this age?

What actually helped?