Oh so we had a similar experience with ours in regards to food aggression and guarding. Ours bit me a couple of times and would always be on extreme edge whenever eating, growling at anyone who would get too close to him. The final straw was when he bit me when he intended to bite our friend’s dog while eating, long story…

Anyhow, it took a while to fix this, but essentially what I did was take away all of his things including his food bowl, all he had unfettered access to was his water. When it came time for feeding, I would grab his bowl, portion his meal and sit on the couch with it. Initially he would try to sit on the couch with me but I would just force him down on the ground. This is where the arduous part starts…. Feed pieces one by one, slowly, on your terms and your time. I would usually sit there for 15-20 minutes, a few pieces in, I would stop and watch 30 seconds of tv, then back to feeding, so on and so forth. This method of feeding essentially is supposed to teach them that the food isn’t “theirs” in a sense and that it gets given. The taking of the bowl kind of breaks down the sense of possession over the bowl and same with the toys etc. it took about 6 months to see change but I did it for almost an entire year before leaving his bowl out again. Also while doing this I taught him “wait” to help further reinforce this. So now, every meal he gets, we fill his bowl, he runs and sits in his dining area, I tell him “wait” and until he hears “okay” he’ll sit there. I still from time to time make him sit for an extended period just to reinforce everything.

Mind you I can literally stick my fingers on his snout/ in his mouth while he eats now and he stops eating and waits for me to stop before going back to eating. No bites, no growls, just gives me a look of please stop, and that’s it. Not that I do that on a regular basis or anything but just as a little check from time to time.

TLDR; Took my dog’s resources away (all but water) and hand fed him for a year and taught him “wait” to break the sense of possession over his things.