I might sound quite blunt as I tend to be honest about issues like this so apologies in advance if it comes across as a bit short - but it is coming from a place of trying to help, I just think honesty is important.
The bad news: the main issue here if your Golden Retriever puppy wasn't well bred and almost certainly had issues around food before you ever re-homed him. Champion bloodlines and good health do not a good breeder maker. (Were the parents heart tested, out of interest?)
Resource guarding is not unusual with poorly bred Golden Retrievers combined with novice owners as it's hard to spot the warning signs. It's not 'lines' as a PP has suggested where Golden Retrievers have shown guarding issues - it's not hereditary like PRA or dysplasia. It's all down to the breeder or the owner. It's only hereditary in so much as bad breeders breed defective dogs so every dog they breed is likely to have issues.
Sometimes it does come from nowhere, but with a puppy as young as yours...it's a breeder issue. I imagine the instance at the vet just reinforced the behaviour the dog had already learnt with his littermates. So, unfortunately, you didn't start with a 'blank slate' - you started with a puppy who already had issues around food.
The good news: this is only food. No dogs need long lasting treats. They are unnecessary. They do absolutely nothing for the dogs health, wellbeing or mental enrichment. They are a tool used by owners to keep the dog distracted. Training is always a better option (10 minutes 4-5 times a day instead of any 'mental enrichment toys').
So, the answer is: no more chews. No lick matts. No kongs. No pigs ear. Nothing that isn't either a quick treat (bit of chicken etc) when he's done something good, or his dinner. If you continue to give him things that he could guard, it will never improve and it could escalate - including to resource guarding people, toys or other things.
Honestly though (and I genuinely don't say this lightly), but resource guarding in a dog this young, when there's a novice owner at the wheel, is going to be a challenge. I am really sorry because whilst resource guarding can be managed, you're forever going to be on your toes and waiting for something to happen. Resource guarding is one of the hardest things to manage when it comes to dog behaviour, and particularly with Golden Retrievers.
Edit: Just to say, I know the breed really well (I currently have 5 Golden Retrievers and have been breeding for 21 years and my mum bred for 30 years before me, I also do re-homing for my local rescue and show). So the above is based on decades of experience of the breed - not just me mouthing off without knowing what I'm saying! (Which sometimes it does come across as)😃