Professional advice is the best route to go. In the meantime there is a wonderful YT channel called “No Bad Dogs” out of NYC I believe. Their approach is full-spectrum regarding positive reinforcement, positive punishment, etc. Also, one of the main ways I work with my (non-abused puppies & not a professional) with resource guarding is as follows: grab a low-value and high-value item. Give the dog the low-value item, tell them “out” after a short time (no touching/forcing), the moment they drop it say “yes” and give them the high-value item. Do this randomly, short and sweet. Best of luck.