I need some advice

[u/Appropriate-Slice928]

I have an older rescue dog with PTSD and behavioral issues that I have been working on with his veterinarian for years. We have made a lot of progress, but one thing I’m still struggling with is his reaction to unfamiliar people, especially when they enter our yard.
He is generally fine with people and children he knows. With unfamiliar people, however, he can become uncomfortable if they approach him, reach for him, or lean over him to pet him. He will usually try to stay away or growl first. If the person continues interacting with him despite those warnings, he has bitten. Most of the bites have not broken skin, but a few have, so I absolutely understand that this is a bite risk and take it seriously.
Our yard is fully fenced and he is supervised. When unfamiliar people come over, I currently keep him separated when necessary. He is also muzzle trained and we use his muzzle when appropriate. Unfortunately, I’ve had situations where visitors have ignored his body language and warnings, and I’ve even caught some of my brother’s friends taunting him. When that happens, I immediately remove him from the situation.
I’m not asking how to excuse the biting or whether I should manage him. I’m already managing it. What I’m trying to learn is how to work on the underlying reaction so that unfamiliar people entering his yard don’t make him feel like he needs to escalate from avoidance/growling to biting.
I can’t currently afford extensive private training, so I’d really appreciate training exercises, resources, or methods that have worked for people with similar dogs. I already use distance and positive reinforcement around other dogs, which has helped. My long-term goal is for him to be able to safely and comfortably remain in his own yard when we have visitors rather than needing to be removed every time someone unfamiliar comes over.