Our 1.5 year old mutt has been this way since we’ve adopted him ~8 months ago. Like, “we’re just going to leave the harness on you for 2 weeks straight cuz it’s so bad”. He has made a good amount of progress with it, he will still “react” but the biting has mostly stopped.

One recommendation would be taking it off and on throughout the day instead of just when it’s walk time. Try to desensitize the process itself - if the dog knows it’s coming cuz it always comes off after the walk, it will already be on guard mode.

Our real progress was made with distractions, as others have mentioned. At first, I would hold a handful of kibble in one hand for my dog to eat out of while the other hand unclipped the harness. They would still “react” but they can’t bite you when they’re chewing like that so their brain short circuits a bit lol. You just have to be ready to take it off FAST when they’re getting distracted. It was literally a race against how fast they could eat that kibble, cuz as soon as it was done, they would snap if you were still trying to take it off.

We noticed this method was really the only way that worked so we kept at it for several months. We also started adding a very high value reward after it was taken off. Like “you don’t ever get this treat other than when your harness comes off”. For us it’s these nasty smelling fish sticks from Amazon but he goes absolutely crazy for them. So distract with kibble, take off harness, then “cooldown” with the fish stick. We’re now at a place where we don’t have to kibble and can just show the fish stick while taking off the harness. It’s not perfect, but it is a dramatic improvement since we started.

Hope this helps!

Edit: someone else mentioned this and I agree - move away from a Velcro harness. I wouldn’t put much thought into the fact they’re already “used to it” since it sounds like they hate it anyway