I have had this same issue with Gunther, although maybe not as extreme.
He will however get snarky with other dogs over toys and water at the dog park if he doesn't know them. He will also on occasion get grumpy if he is near me and another dog comes up, although he is getting much better about this.
What I do is tell him to "leave it". If he is drinking water and another dog comes up and i see him lower his head and eyeball the other dog...........
.....i will tell him to "leave it". He knows that means to walk away. Same thing for a toy...........
which is usually a stick.
As for guarding me, his word is "out". I use it at home when I want one of the dogs to leave the room. Although there isn't a "room" at the dog park, when I tell him "out" and point away from me, he knows he is meant to move away from me, although reluctantly. And he usually reappears a few seconds later........m
omma's boy that he is.
so that command often gets repeated as needed.
If neither of these is working and he gets snarky multiple times, he is put in a time out, at least 10-15 ft from where i am standing. To him this is torture so the point gets across quickly.......
.....especiall
y if I face him the opposite direction (although he usually turns himself around so he can at least see me).
Good luck! It can be difficult with dogs with a protective nature, but progress can be made.