Resource Guarding

Hello! I've read a few things on resource guarding in the last couple of days but none of them seem to meet my dog's situation. He seems to be a full great pyrenees (I don't have papers for him, he was just a lil puppy off a sheep farm, I've had him since he was 8 weeks) and he's recently 4 years old.
He's never resource guarded until last week, but it's been a constant issue now. He grew up with cats, dogs, cows, and frequent children. He has no issue with human hands, other dogs, the cats, or even the chickens being in his bowls. While he's eating/drinking, while he's not eating, empty, full - he isn't bothered by anything except for the goats.
We have recently acquired three baby goats. He loves them, lets them jump on him, he protects them as he should the whole thing. We moved two of the goats into the barn now that it's warming up and it was only after they were put outside that he's started having this issue.
He doesn't mind that they're eating the hay but he'll keep a very close eye on them. If they go near the water bucket or near their oat bucket, he'll growl and snap (he seems to be careful not to actually bite despite being in range to. Just a big snap of his teeth. Regardless if these buckets are empty or full.)

They had no issue sharing the water bucket until the goats were moved outside, though. They learned to drink from the bucket watching him. It started with guarding the sweet oats but its now escalated to guarding the water bucket, too.

I proposed to my husband that it may have started as a diet issue. He's been acting strangely lately trying to eat dirt more than most dogs normally would - our vet said he's just being a dog and has a sensitive belly. He has an allergy to either beef or wheat and our vet suggested we just remove both from his diet to be safe. He also tries to get into the chicken food often, but doesn't guard the chickens from it. He just eats alongside them, as well as the chickens getting into the sweet oats - he just lets them nibble along with him. I try not to let him eat the feed or oats but I also can't be outside all the time and can't keep him cooped up inside all the time.
He's on a chicken and rice kibble diet and we include cooked farm eggs into his meals. We plan on getting the animals all onto a farm-raised diet when our farm gets to a healthy pace. As for now, he doesn't mind the other dog taking some of his food, my husband and I can put our hands in his bowl, take food out and feed it to the dogs or feed a little piece of egg to the cat right in front of him and he just waits patiently until our hands are out of the bowl and munches along. The other animals he'll let take some nibbles while he's eating.

Sorry for repeating myself so often throughout this, it's just so boggling.