Hi Emtnicki,
Melissaandcrew gave you excellent advice. There were a few papers published that concluded lower protein diets decreased certain types of aggression. In my opinion there were some problems in how the studies were run and I don’t think the results were really valid. If you want to continue with lower protein foods you may want to take a look at “senior” diets as many companies lower the protein in their foods marketed to that age group.
In general when working with resource guarders if the item is not dangerous to the dog and not vitally important to you then let the dog keep it. Only remove the item after the dog has abandoned it and it and there is a solid barrier between you and the dog.
You don’t want the dog to pair getting cheese or hot dog or a walk etc with stealing and possessing an item. This is why Melissa recommended establishing the cue “let’s get cheese” independent of when the dog has something. There should be many “let’s get cheese” cues without needing to recover an item compared to using it to recover an item.
For professionals to assist you I’d look to see if there are any veterinary behaviorists in your area
http://www.dacvb.org or Karen Pryor Academy trainers https://www.karenpryoracademy.com/find-a-trainer
